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x86/mm: Convert mmu context ia32_compat into a proper flags field 

The ia32_compat attribute is a weird thing.  It mirrors TIF_IA32 and
TIF_X32 and is used only in two very unrelated places: (1) to decide if
the vsyscall page is accessible (2) for uprobes to find whether the
patched instruction is 32 or 64 bit.

In preparation to remove the TIF flags, a new mechanism is required for
ia32_compat, but given its odd semantics, adding a real flags field which
configures these specific behaviours is the best option.

So, set_personality_x64() can ask for the vsyscall page, which is not
available in x32/ia32 and set_personality_ia32() can configure the uprobe
code as needed.

uprobe cannot rely on other methods like user_64bit_mode() to decide how
to patch, so it needs some specific flag like this.

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Andy Lutomirski<luto@kernel.org>
Link: https://lore.kernel.org/r/20201004032536.1229030-10-krisman@collabora.com
This commit is contained in:
Gabriel Krisman Bertazi 2020-10-03 23:25:35 -04:00 committed by Thomas Gleixner
parent 3316ec8ccd
commit ff170cd059
4 changed files with 20 additions and 10 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
arch/x86/entry/vsyscall/vsyscall_64.c
View File

@ -316,7 +316,7 @@ static struct vm_area_struct gate_vma __ro_after_init = {
struct vm_area_struct *get_gate_vma(struct mm_struct *mm)
{
#ifdef CONFIG_COMPAT
if (!mm || mm->context.ia32_compat)
if (!mm || !(mm->context.flags & MM_CONTEXT_HAS_VSYSCALL))
return NULL;
#endif
if (vsyscall_mode == NONE)

9
arch/x86/include/asm/mmu.h
View File

@ -6,6 +6,12 @@
#include <linux/rwsem.h>
#include <linux/mutex.h>
#include <linux/atomic.h>
#include <linux/bits.h>
/* Uprobes on this MM assume 32-bit code */
#define MM_CONTEXT_UPROBE_IA32 BIT(0)
/* vsyscall page is accessible on this MM */
#define MM_CONTEXT_HAS_VSYSCALL BIT(1)
/*
* x86 has arch-specific MMU state beyond what lives in mm_struct.
@ -33,8 +39,7 @@ typedef struct {
#endif
#ifdef CONFIG_X86_64
/* True if mm supports a task running in 32 bit compatibility mode. */
unsigned short ia32_compat;
unsigned short flags;
#endif
struct mutex lock;

2
arch/x86/include/asm/mmu_context.h
View File

@ -177,7 +177,7 @@ static inline void arch_exit_mmap(struct mm_struct *mm)
static inline bool is_64bit_mm(struct mm_struct *mm)
{
return !IS_ENABLED(CONFIG_IA32_EMULATION) ||
!(mm->context.ia32_compat == TIF_IA32);
!(mm->context.flags & MM_CONTEXT_UPROBE_IA32);
}
#else
static inline bool is_64bit_mm(struct mm_struct *mm)

17
arch/x86/kernel/process_64.c
View File

@ -646,10 +646,8 @@ void set_personality_64bit(void)
/* Pretend that this comes from a 64bit execve */
task_pt_regs(current)->orig_ax = __NR_execve;
current_thread_info()->status &= ~TS_COMPAT;
/* Ensure the corresponding mm is not marked. */
if (current->mm)
current->mm->context.ia32_compat = 0;
current->mm->context.flags = MM_CONTEXT_HAS_VSYSCALL;
/* TBD: overwrites user setup. Should have two bits.
But 64bit processes have always behaved this way,
@ -664,7 +662,8 @@ static void __set_personality_x32(void)
clear_thread_flag(TIF_IA32);
set_thread_flag(TIF_X32);
if (current->mm)
current->mm->context.ia32_compat = TIF_X32;
current->mm->context.flags = 0;
current->personality &= ~READ_IMPLIES_EXEC;
/*
* in_32bit_syscall() uses the presence of the x32 syscall bit
@ -684,8 +683,14 @@ static void __set_personality_ia32(void)
#ifdef CONFIG_IA32_EMULATION
set_thread_flag(TIF_IA32);
clear_thread_flag(TIF_X32);
if (current->mm)
current->mm->context.ia32_compat = TIF_IA32;
if (current->mm) {
/*
* uprobes applied to this MM need to know this and
* cannot use user_64bit_mode() at that time.
*/
current->mm->context.flags = MM_CONTEXT_UPROBE_IA32;
}
current->personality |= force_personality32;
/* Prepare the first "return" to user space */
task_pt_regs(current)->orig_ax = __NR_ia32_execve;