6.6-stable review patch. If anyone has any objections, please let me know.
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From: Andrei Matei andreimatei1@gmail.com
[ Upstream commit a833a17aeac73b33f79433d7cee68d5cafd71e4f ]
This patch fixes a bug around the verification of possibly-zero-sized stack accesses. When the access was done through a var-offset stack pointer, check_stack_access_within_bounds was incorrectly computing the maximum-offset of a zero-sized read to be the same as the register's min offset. Instead, we have to take in account the register's maximum possible value. The patch also simplifies how the max offset is checked; the check is now simpler than for min offset.
The bug was allowing accesses to erroneously pass the check_stack_access_within_bounds() checks, only to later crash in check_stack_range_initialized() when all the possibly-affected stack slots are iterated (this time with a correct max offset). check_stack_range_initialized() is relying on check_stack_access_within_bounds() for its accesses to the stack-tracking vector to be within bounds; in the case of zero-sized accesses, we were essentially only verifying that the lowest possible slot was within bounds. We would crash when the max-offset of the stack pointer was >= 0 (which shouldn't pass verification, and hopefully is not something anyone's code attempts to do in practice).
Thanks Hao for reporting!
Fixes: 01f810ace9ed3 ("bpf: Allow variable-offset stack access") Reported-by: Hao Sun sunhao.th@gmail.com Signed-off-by: Andrei Matei andreimatei1@gmail.com Signed-off-by: Andrii Nakryiko andrii@kernel.org Acked-by: Eduard Zingerman eddyz87@gmail.com Acked-by: Andrii Nakryiko andrii@kernel.org Link: https://lore.kernel.org/bpf/20231207041150.229139-2-andreimatei1@gmail.com
Closes: https://lore.kernel.org/bpf/CACkBjsZGEUaRCHsmaX=h-efVogsRfK1FPxmkgb0Os_frnHi... Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/bpf/verifier.c | 14 ++++---------- 1 file changed, 4 insertions(+), 10 deletions(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 24152ac6a393..76834ecc59a9 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -6414,10 +6414,7 @@ static int check_stack_access_within_bounds(
if (tnum_is_const(reg->var_off)) { min_off = reg->var_off.value + off; - if (access_size > 0) - max_off = min_off + access_size - 1; - else - max_off = min_off; + max_off = min_off + access_size; } else { if (reg->smax_value >= BPF_MAX_VAR_OFF || reg->smin_value <= -BPF_MAX_VAR_OFF) { @@ -6426,15 +6423,12 @@ static int check_stack_access_within_bounds( return -EACCES; } min_off = reg->smin_value + off; - if (access_size > 0) - max_off = reg->smax_value + off + access_size - 1; - else - max_off = min_off; + max_off = reg->smax_value + off + access_size; }
err = check_stack_slot_within_bounds(min_off, state, type); - if (!err) - err = check_stack_slot_within_bounds(max_off, state, type); + if (!err && max_off > 0) + err = -EINVAL; /* out of stack access into non-negative offsets */
if (err) { if (tnum_is_const(reg->var_off)) {