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linux-stable/arch/arm/kernel/unwind.c
Ard Biesheuvel f6b8e3526f ARM: unwind: only permit stack switch when unwinding call_with_stack() 
Commit b6506981f8 ("ARM: unwind: support unwinding across multiple
stacks") updated the logic in the ARM unwinder to widen the bounds
within which SP is assumed to be valid, in order to allow the unwind to
traverse from the IRQ stack to the task stack. This is necessary, as
otherwise, unwinds started from the IRQ stack would terminate in the IRQ
exception handler, making stacktraces substantially less useful.

This turns out to be a mistake, as it breaks asynchronous unwinding
across exceptions, when the exception is taken before the stack frame is
consistent with the unwind info. For instance, in the following
backtrace:

  ...
   generic_handle_arch_irq from call_with_stack+0x18/0x20
   call_with_stack from __irq_svc+0x80/0x98
  Exception stack(0xc7093e20 to 0xc7093e68)
  3e20: b6a94a88 c7093ea0 00000008 00000000 c7093ea0 b7e127d0 00000051 c9220000
  3e40: b6a94a88 b6a94a88 00000004 0002b000 0036b570 c7093e70 c040ca2c c0994a90
  3e60: 20070013 ffffffff
   __irq_svc from __copy_to_user_std+0x20/0x378
  ...

we need to apply the following unwind directives:

  0xc099720c <__copy_to_user_std+0x1c>: @0xc295d1d4
    Compact model index: 1
    0x9b      vsp = r11
    0xb1 0x0d pop {r0, r2, r3}
    0x84 0x81 pop {r4, r11, r14}
    0xb0      finish

which tell us to switch to the frame pointer register R11 and proceed
with the unwind from that. However, having been interrupted 0x20 bytes
into the function:

  c09971f0 <__copy_to_user_std>:
  c09971f0:       e59f3350        ldr     r3, [pc, #848]
  c09971f4:       e243c001        sub     ip, r3, #1
  c09971f8:       e05cc000        subs    ip, ip, r0
  c09971fc:       228cc001        addcs   ip, ip, #1
  c0997200:       205cc002        subscs  ip, ip, r2
  c0997204:       33a00000        movcc   r0, #0
  c0997208:       e320f014        csdb
  c099720c:       e3a03000        mov     r3, #0
  c0997210:       e92d481d        push    {r0, r2, r3, r4, fp, lr}
  c0997214:       e1a0b00d        mov     fp, sp
  c0997218:       e2522004        subs    r2, r2, #4

the value for R11 recovered from the previous frame (__irq_svc) will be
a snapshot of its value before the exception was taken (0x0002b000),
which occurred at address __copy_to_user_std+0x20 (0xc0997210), when R11
had not been assigned its value yet.

This means we can never assume that the SP values recovered from the
stack or from the frame pointer are ever safe to use, given the need to
do asynchronous unwinding, and the only robust approach is to revert to
the previous approach, which is to derive bounds for SP based on the
initial value, and never update them.

We can make an exception, though: now that the IRQ stack switch is
guaranteed to occur in call_with_stack(), we can implement a special
case for this function, and use a different set of bounds based on the
knowledge that it will always unwind from R11 rather than SP. As
call_with_stack() is a hand-rolled assembly routine, this is guaranteed
to remain that way.

So let's do a partial revert of b6506981f8, and drop all manipulations
for sp_low and sp_high based on the information collected during the
unwind itself. To support call_with_stack(), set sp_low and sp_high
explicitly to values derived from R11 when we unwind that function.

The only downside is that, while unwinding an overflow of the vmap'ed
stack will work fine as before, we will no longer be able to produce a
backtrace that unwinds the overflow stack itself across the exception
that was raised due to the faulting access to the guard region. However,
this only affects exceptions caused by problems in the stack overflow
handling code itself, in which case the remaining backtrace is not that
relevant.

Fixes: b6506981f8 ("ARM: unwind: support unwinding across multiple stacks")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
2022-03-11 13:01:00 +00:00

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// SPDX-License-Identifier: GPL-2.0-only
/*
* arch/arm/kernel/unwind.c
*
* Copyright (C) 2008 ARM Limited
*
* Stack unwinding support for ARM
*
* An ARM EABI version of gcc is required to generate the unwind
* tables. For information about the structure of the unwind tables,
* see "Exception Handling ABI for the ARM Architecture" at:
*
* http://infocenter.arm.com/help/topic/com.arm.doc.subset.swdev.abi/index.html
*/
#ifndef __CHECKER__
#if !defined (__ARM_EABI__)
#warning Your compiler does not have EABI support.
#warning ARM unwind is known to compile only with EABI compilers.
#warning Change compiler or disable ARM_UNWIND option.
#endif
#endif /* __CHECKER__ */
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/export.h>
#include <linux/sched.h>
#include <linux/slab.h>
#include <linux/spinlock.h>
#include <linux/list.h>
#include <asm/stacktrace.h>
#include <asm/traps.h>
#include <asm/unwind.h>
#include "reboot.h"
/* Dummy functions to avoid linker complaints */
void __aeabi_unwind_cpp_pr0(void)
{
};
EXPORT_SYMBOL(__aeabi_unwind_cpp_pr0);
void __aeabi_unwind_cpp_pr1(void)
{
};
EXPORT_SYMBOL(__aeabi_unwind_cpp_pr1);
void __aeabi_unwind_cpp_pr2(void)
{
};
EXPORT_SYMBOL(__aeabi_unwind_cpp_pr2);
struct unwind_ctrl_block {
unsigned long vrs[16]; /* virtual register set */
const unsigned long *insn; /* pointer to the current instructions word */
unsigned long sp_high; /* highest value of sp allowed */
unsigned long *lr_addr; /* address of LR value on the stack */
/*
* 1 : check for stack overflow for each register pop.
* 0 : save overhead if there is plenty of stack remaining.
*/
int check_each_pop;
int entries; /* number of entries left to interpret */
int byte; /* current byte number in the instructions word */
};
enum regs {
#ifdef CONFIG_THUMB2_KERNEL
FP = 7,
#else
FP = 11,
#endif
SP = 13,
LR = 14,
PC = 15
};
extern const struct unwind_idx __start_unwind_idx[];
static const struct unwind_idx *__origin_unwind_idx;
extern const struct unwind_idx __stop_unwind_idx[];
static DEFINE_RAW_SPINLOCK(unwind_lock);
static LIST_HEAD(unwind_tables);
/* Convert a prel31 symbol to an absolute address */
#define prel31_to_addr(ptr) \
({ \
/* sign-extend to 32 bits */ \
long offset = (((long)*(ptr)) << 1) >> 1; \
(unsigned long)(ptr) + offset; \
})
/*
* Binary search in the unwind index. The entries are
* guaranteed to be sorted in ascending order by the linker.
*
* start = first entry
* origin = first entry with positive offset (or stop if there is no such entry)
* stop - 1 = last entry
*/
static const struct unwind_idx *search_index(unsigned long addr,
const struct unwind_idx *start,
const struct unwind_idx *origin,
const struct unwind_idx *stop)
{
unsigned long addr_prel31;
pr_debug("%s(%08lx, %p, %p, %p)\n",
__func__, addr, start, origin, stop);
/*
* only search in the section with the matching sign. This way the
* prel31 numbers can be compared as unsigned longs.
*/
if (addr < (unsigned long)start)
/* negative offsets: [start; origin) */
stop = origin;
else
/* positive offsets: [origin; stop) */
start = origin;
/* prel31 for address relavive to start */
addr_prel31 = (addr - (unsigned long)start) & 0x7fffffff;
while (start < stop - 1) {
const struct unwind_idx *mid = start + ((stop - start) >> 1);
/*
* As addr_prel31 is relative to start an offset is needed to
* make it relative to mid.
*/
if (addr_prel31 - ((unsigned long)mid - (unsigned long)start) <
mid->addr_offset)
stop = mid;
else {
/* keep addr_prel31 relative to start */
addr_prel31 -= ((unsigned long)mid -
(unsigned long)start);
start = mid;
}
}
if (likely(start->addr_offset <= addr_prel31))
return start;
else {
pr_warn("unwind: Unknown symbol address %08lx\n", addr);
return NULL;
}
}
static const struct unwind_idx *unwind_find_origin(
const struct unwind_idx *start, const struct unwind_idx *stop)
{
pr_debug("%s(%p, %p)\n", __func__, start, stop);
while (start < stop) {
const struct unwind_idx *mid = start + ((stop - start) >> 1);
if (mid->addr_offset >= 0x40000000)
/* negative offset */
start = mid + 1;
else
/* positive offset */
stop = mid;
}
pr_debug("%s -> %p\n", __func__, stop);
return stop;
}
static const struct unwind_idx *unwind_find_idx(unsigned long addr)
{
const struct unwind_idx *idx = NULL;
unsigned long flags;
pr_debug("%s(%08lx)\n", __func__, addr);
if (core_kernel_text(addr)) {
if (unlikely(!__origin_unwind_idx))
__origin_unwind_idx =
unwind_find_origin(__start_unwind_idx,
__stop_unwind_idx);
/* main unwind table */
idx = search_index(addr, __start_unwind_idx,
__origin_unwind_idx,
__stop_unwind_idx);
} else {
/* module unwind tables */
struct unwind_table *table;
raw_spin_lock_irqsave(&unwind_lock, flags);
list_for_each_entry(table, &unwind_tables, list) {
if (addr >= table->begin_addr &&
addr < table->end_addr) {
idx = search_index(addr, table->start,
table->origin,
table->stop);
/* Move-to-front to exploit common traces */
list_move(&table->list, &unwind_tables);
break;
}
}
raw_spin_unlock_irqrestore(&unwind_lock, flags);
}
pr_debug("%s: idx = %p\n", __func__, idx);
return idx;
}
static unsigned long unwind_get_byte(struct unwind_ctrl_block *ctrl)
{
unsigned long ret;
if (ctrl->entries <= 0) {
pr_warn("unwind: Corrupt unwind table\n");
return 0;
}
ret = (*ctrl->insn >> (ctrl->byte * 8)) & 0xff;
if (ctrl->byte == 0) {
ctrl->insn++;
ctrl->entries--;
ctrl->byte = 3;
} else
ctrl->byte--;
return ret;
}
/* Before poping a register check whether it is feasible or not */
static int unwind_pop_register(struct unwind_ctrl_block *ctrl,
unsigned long **vsp, unsigned int reg)
{
if (unlikely(ctrl->check_each_pop))
if (*vsp >= (unsigned long *)ctrl->sp_high)
return -URC_FAILURE;
/* Use READ_ONCE_NOCHECK here to avoid this memory access
* from being tracked by KASAN.
*/
ctrl->vrs[reg] = READ_ONCE_NOCHECK(*(*vsp));
if (reg == 14)
ctrl->lr_addr = *vsp;
(*vsp)++;
return URC_OK;
}
/* Helper functions to execute the instructions */
static int unwind_exec_pop_subset_r4_to_r13(struct unwind_ctrl_block *ctrl,
unsigned long mask)
{
unsigned long *vsp = (unsigned long *)ctrl->vrs[SP];
int load_sp, reg = 4;
load_sp = mask & (1 << (13 - 4));
while (mask) {
if (mask & 1)
if (unwind_pop_register(ctrl, &vsp, reg))
return -URC_FAILURE;
mask >>= 1;
reg++;
}
if (!load_sp) {
ctrl->vrs[SP] = (unsigned long)vsp;
}
return URC_OK;
}
static int unwind_exec_pop_r4_to_rN(struct unwind_ctrl_block *ctrl,
unsigned long insn)
{
unsigned long *vsp = (unsigned long *)ctrl->vrs[SP];
int reg;
/* pop R4-R[4+bbb] */
for (reg = 4; reg <= 4 + (insn & 7); reg++)
if (unwind_pop_register(ctrl, &vsp, reg))
return -URC_FAILURE;
if (insn & 0x8)
if (unwind_pop_register(ctrl, &vsp, 14))
return -URC_FAILURE;
ctrl->vrs[SP] = (unsigned long)vsp;
return URC_OK;
}
static int unwind_exec_pop_subset_r0_to_r3(struct unwind_ctrl_block *ctrl,
unsigned long mask)
{
unsigned long *vsp = (unsigned long *)ctrl->vrs[SP];
int reg = 0;
/* pop R0-R3 according to mask */
while (mask) {
if (mask & 1)
if (unwind_pop_register(ctrl, &vsp, reg))
return -URC_FAILURE;
mask >>= 1;
reg++;
}
ctrl->vrs[SP] = (unsigned long)vsp;
return URC_OK;
}
/*
* Execute the current unwind instruction.
*/
static int unwind_exec_insn(struct unwind_ctrl_block *ctrl)
{
unsigned long insn = unwind_get_byte(ctrl);
int ret = URC_OK;
pr_debug("%s: insn = %08lx\n", __func__, insn);
if ((insn & 0xc0) == 0x00)
ctrl->vrs[SP] += ((insn & 0x3f) << 2) + 4;
else if ((insn & 0xc0) == 0x40) {
ctrl->vrs[SP] -= ((insn & 0x3f) << 2) + 4;
} else if ((insn & 0xf0) == 0x80) {
unsigned long mask;
insn = (insn << 8) | unwind_get_byte(ctrl);
mask = insn & 0x0fff;
if (mask == 0) {
pr_warn("unwind: 'Refuse to unwind' instruction %04lx\n",
insn);
return -URC_FAILURE;
}
ret = unwind_exec_pop_subset_r4_to_r13(ctrl, mask);
if (ret)
goto error;
} else if ((insn & 0xf0) == 0x90 &&
(insn & 0x0d) != 0x0d) {
ctrl->vrs[SP] = ctrl->vrs[insn & 0x0f];
} else if ((insn & 0xf0) == 0xa0) {
ret = unwind_exec_pop_r4_to_rN(ctrl, insn);
if (ret)
goto error;
} else if (insn == 0xb0) {
if (ctrl->vrs[PC] == 0)
ctrl->vrs[PC] = ctrl->vrs[LR];
/* no further processing */
ctrl->entries = 0;
} else if (insn == 0xb1) {
unsigned long mask = unwind_get_byte(ctrl);
if (mask == 0 || mask & 0xf0) {
pr_warn("unwind: Spare encoding %04lx\n",
(insn << 8) | mask);
return -URC_FAILURE;
}
ret = unwind_exec_pop_subset_r0_to_r3(ctrl, mask);
if (ret)
goto error;
} else if (insn == 0xb2) {
unsigned long uleb128 = unwind_get_byte(ctrl);
ctrl->vrs[SP] += 0x204 + (uleb128 << 2);
} else {
pr_warn("unwind: Unhandled instruction %02lx\n", insn);
return -URC_FAILURE;
}
pr_debug("%s: fp = %08lx sp = %08lx lr = %08lx pc = %08lx\n", __func__,
ctrl->vrs[FP], ctrl->vrs[SP], ctrl->vrs[LR], ctrl->vrs[PC]);
error:
return ret;
}
/*
* Unwind a single frame starting with *sp for the symbol at *pc. It
* updates the *pc and *sp with the new values.
*/
int unwind_frame(struct stackframe *frame)
{
const struct unwind_idx *idx;
struct unwind_ctrl_block ctrl;
unsigned long sp_low;
/* store the highest address on the stack to avoid crossing it*/
sp_low = frame->sp;
ctrl.sp_high = ALIGN(sp_low - THREAD_SIZE, THREAD_ALIGN)
+ THREAD_SIZE;
pr_debug("%s(pc = %08lx lr = %08lx sp = %08lx)\n", __func__,
frame->pc, frame->lr, frame->sp);
idx = unwind_find_idx(frame->pc);
if (!idx) {
if (frame->pc && kernel_text_address(frame->pc))
pr_warn("unwind: Index not found %08lx\n", frame->pc);
return -URC_FAILURE;
}
ctrl.vrs[FP] = frame->fp;
ctrl.vrs[SP] = frame->sp;
ctrl.vrs[LR] = frame->lr;
ctrl.vrs[PC] = 0;
if (idx->insn == 1)
/* can't unwind */
return -URC_FAILURE;
else if (frame->pc == prel31_to_addr(&idx->addr_offset)) {
/*
* Unwinding is tricky when we're halfway through the prologue,
* since the stack frame that the unwinder expects may not be
* fully set up yet. However, one thing we do know for sure is
* that if we are unwinding from the very first instruction of
* a function, we are still effectively in the stack frame of
* the caller, and the unwind info has no relevance yet.
*/
if (frame->pc == frame->lr)
return -URC_FAILURE;
frame->pc = frame->lr;
return URC_OK;
} else if ((idx->insn & 0x80000000) == 0)
/* prel31 to the unwind table */
ctrl.insn = (unsigned long *)prel31_to_addr(&idx->insn);
else if ((idx->insn & 0xff000000) == 0x80000000)
/* only personality routine 0 supported in the index */
ctrl.insn = &idx->insn;
else {
pr_warn("unwind: Unsupported personality routine %08lx in the index at %p\n",
idx->insn, idx);
return -URC_FAILURE;
}
/* check the personality routine */
if ((*ctrl.insn & 0xff000000) == 0x80000000) {
ctrl.byte = 2;
ctrl.entries = 1;
} else if ((*ctrl.insn & 0xff000000) == 0x81000000) {
ctrl.byte = 1;
ctrl.entries = 1 + ((*ctrl.insn & 0x00ff0000) >> 16);
} else {
pr_warn("unwind: Unsupported personality routine %08lx at %p\n",
*ctrl.insn, ctrl.insn);
return -URC_FAILURE;
}
ctrl.check_each_pop = 0;
if (prel31_to_addr(&idx->addr_offset) == (u32)&call_with_stack) {
/*
* call_with_stack() is the only place where we permit SP to
* jump from one stack to another, and since we know it is
* guaranteed to happen, set up the SP bounds accordingly.
*/
sp_low = frame->fp;
ctrl.sp_high = ALIGN(frame->fp, THREAD_SIZE);
}
while (ctrl.entries > 0) {
int urc;
if ((ctrl.sp_high - ctrl.vrs[SP]) < sizeof(ctrl.vrs))
ctrl.check_each_pop = 1;
urc = unwind_exec_insn(&ctrl);
if (urc < 0)
return urc;
if (ctrl.vrs[SP] < sp_low || ctrl.vrs[SP] > ctrl.sp_high)
return -URC_FAILURE;
}
if (ctrl.vrs[PC] == 0)
ctrl.vrs[PC] = ctrl.vrs[LR];
/* check for infinite loop */
if (frame->pc == ctrl.vrs[PC] && frame->sp == ctrl.vrs[SP])
return -URC_FAILURE;
frame->fp = ctrl.vrs[FP];
frame->sp = ctrl.vrs[SP];
frame->lr = ctrl.vrs[LR];
frame->pc = ctrl.vrs[PC];
frame->lr_addr = ctrl.lr_addr;
return URC_OK;
}
void unwind_backtrace(struct pt_regs *regs, struct task_struct *tsk,
const char *loglvl)
{
struct stackframe frame;
pr_debug("%s(regs = %p tsk = %p)\n", __func__, regs, tsk);
if (!tsk)
tsk = current;
if (regs) {
arm_get_current_stackframe(regs, &frame);
/* PC might be corrupted, use LR in that case. */
if (!kernel_text_address(regs->ARM_pc))
frame.pc = regs->ARM_lr;
} else if (tsk == current) {
frame.fp = (unsigned long)__builtin_frame_address(0);
frame.sp = current_stack_pointer;
frame.lr = (unsigned long)__builtin_return_address(0);
/* We are saving the stack and execution state at this
* point, so we should ensure that frame.pc is within
* this block of code.
*/
here:
frame.pc = (unsigned long)&&here;
} else {
/* task blocked in __switch_to */
frame.fp = thread_saved_fp(tsk);
frame.sp = thread_saved_sp(tsk);
/*
* The function calling __switch_to cannot be a leaf function
* so LR is recovered from the stack.
*/
frame.lr = 0;
frame.pc = thread_saved_pc(tsk);
}
while (1) {
int urc;
unsigned long where = frame.pc;
urc = unwind_frame(&frame);
if (urc < 0)
break;
dump_backtrace_entry(where, frame.pc, frame.sp - 4, loglvl);
}
}
struct unwind_table *unwind_table_add(unsigned long start, unsigned long size,
unsigned long text_addr,
unsigned long text_size)
{
unsigned long flags;
struct unwind_table *tab = kmalloc(sizeof(*tab), GFP_KERNEL);
pr_debug("%s(%08lx, %08lx, %08lx, %08lx)\n", __func__, start, size,
text_addr, text_size);
if (!tab)
return tab;
tab->start = (const struct unwind_idx *)start;
tab->stop = (const struct unwind_idx *)(start + size);
tab->origin = unwind_find_origin(tab->start, tab->stop);
tab->begin_addr = text_addr;
tab->end_addr = text_addr + text_size;
raw_spin_lock_irqsave(&unwind_lock, flags);
list_add_tail(&tab->list, &unwind_tables);
raw_spin_unlock_irqrestore(&unwind_lock, flags);
return tab;
}
void unwind_table_del(struct unwind_table *tab)
{
unsigned long flags;
if (!tab)
return;
raw_spin_lock_irqsave(&unwind_lock, flags);
list_del(&tab->list);
raw_spin_unlock_irqrestore(&unwind_lock, flags);
kfree(tab);
}
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