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bpf: Prevent memory disambiguation attack 

commit af86ca4e30 upstream.

Detect code patterns where malicious 'speculative store bypass' can be used
and sanitize such patterns.

 39: (bf) r3 = r10
 40: (07) r3 += -216
 41: (79) r8 = *(u64 *)(r7 +0)   // slow read
 42: (7a) *(u64 *)(r10 -72) = 0  // verifier inserts this instruction
 43: (7b) *(u64 *)(r8 +0) = r3   // this store becomes slow due to r8
 44: (79) r1 = *(u64 *)(r6 +0)   // cpu speculatively executes this load
 45: (71) r2 = *(u8 *)(r1 +0)    // speculatively arbitrary 'load byte'
                                 // is now sanitized

Above code after x86 JIT becomes:
 e5: mov    %rbp,%rdx
 e8: add    $0xffffffffffffff28,%rdx
 ef: mov    0x0(%r13),%r14
 f3: movq   $0x0,-0x48(%rbp)
 fb: mov    %rdx,0x0(%r14)
 ff: mov    0x0(%rbx),%rdi
103: movzbq 0x0(%rdi),%rsi

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
[bwh: Backported to 4.9:
 - Add bpf_verifier_env parameter to check_stack_write()
 - Look up stack slot_types with state->stack_slot_type[] rather than
   state->stack[].slot_type[]
 - Drop bpf_verifier_env argument to verbose()
 - Adjust context]
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Alexei Starovoitov 2018-05-15 09:27:05 -07:00 committed by Greg Kroah-Hartman
parent 62e0865f20
commit def8c1d045
2 changed files with 59 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
include/linux/bpf_verifier.h
View File

@ -71,6 +71,7 @@ struct bpf_insn_aux_data {
enum bpf_reg_type ptr_type; /* pointer type for load/store insns */
struct bpf_map *map_ptr; /* pointer for call insn into lookup_elem */
};
int sanitize_stack_off; /* stack slot to be cleared */
bool seen; /* this insn was processed by the verifier */
};

62
kernel/bpf/verifier.c
View File

@ -540,8 +540,9 @@ static bool is_spillable_regtype(enum bpf_reg_type type)
/* check_stack_read/write functions track spill/fill of registers,
* stack boundary and alignment are checked in check_mem_access()
*/
static int check_stack_write(struct bpf_verifier_state *state, int off,
int size, int value_regno)
static int check_stack_write(struct bpf_verifier_env *env,
struct bpf_verifier_state *state, int off,
int size, int value_regno, int insn_idx)
{
int i, spi = (MAX_BPF_STACK + off) / BPF_REG_SIZE;
/* caller checked that off % size == 0 and -MAX_BPF_STACK <= off < 0,
@ -560,8 +561,32 @@ static int check_stack_write(struct bpf_verifier_state *state, int off,
/* save register state */
state->spilled_regs[spi] = state->regs[value_regno];
for (i = 0; i < BPF_REG_SIZE; i++)
for (i = 0; i < BPF_REG_SIZE; i++) {
if (state->stack_slot_type[MAX_BPF_STACK + off + i] == STACK_MISC &&
!env->allow_ptr_leaks) {
int *poff = &env->insn_aux_data[insn_idx].sanitize_stack_off;
int soff = (-spi - 1) * BPF_REG_SIZE;
/* detected reuse of integer stack slot with a pointer
* which means either llvm is reusing stack slot or
* an attacker is trying to exploit CVE-2018-3639
* (speculative store bypass)
* Have to sanitize that slot with preemptive
* store of zero.
*/
if (*poff && *poff != soff) {
/* disallow programs where single insn stores
* into two different stack slots, since verifier
* cannot sanitize them
*/
verbose("insn %d cannot access two stack slots fp%d and fp%d",
insn_idx, *poff, soff);
return -EINVAL;
}
*poff = soff;
}
state->stack_slot_type[MAX_BPF_STACK + off + i] = STACK_SPILL;
}
} else {
/* regular write of data into stack */
state->spilled_regs[spi] = (struct bpf_reg_state) {};
@ -841,7 +866,8 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
verbose("attempt to corrupt spilled pointer on stack\n");
return -EACCES;
}
err = check_stack_write(state, off, size, value_regno);
err = check_stack_write(env, state, off, size,
value_regno, insn_idx);
} else {
err = check_stack_read(state, off, size, value_regno);
}
@ -3367,6 +3393,34 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
else
continue;
if (type == BPF_WRITE &&
env->insn_aux_data[i + delta].sanitize_stack_off) {
struct bpf_insn patch[] = {
/* Sanitize suspicious stack slot with zero.
* There are no memory dependencies for this store,
* since it's only using frame pointer and immediate
* constant of zero
*/
BPF_ST_MEM(BPF_DW, BPF_REG_FP,
env->insn_aux_data[i + delta].sanitize_stack_off,
0),
/* the original STX instruction will immediately
* overwrite the same stack slot with appropriate value
*/
*insn,
};
cnt = ARRAY_SIZE(patch);
new_prog = bpf_patch_insn_data(env, i + delta, patch, cnt);
if (!new_prog)
return -ENOMEM;
delta += cnt - 1;
env->prog = new_prog;
insn = new_prog->insnsi + i + delta;
continue;
}
if (env->insn_aux_data[i + delta].ptr_type != PTR_TO_CTX)
continue;