Re: [PATCH] bpf: Reset register bounds before narrowing retval range in check_mem_access()

[Jiri Olsa <olsajiri@gmail.com>]

On Wed, Jun 17, 2026 at 12:08:15PM +0000, Tristan Madani wrote:
> From: Tristan Madani <tristan@talencesecurity.com>
> 
> When the BPF verifier processes a context load of an LSM hook return
> value, it calls __mark_reg_s32_range() to narrow the register to the
> hook's valid range. However, __mark_reg_s32_range() intersects the new
> range with the register's existing bounds using max_t()/min_t() rather
> than replacing them.
> 
> If the destination register carries stale bounds from a prior instruction
> (e.g. BPF_MOV64_IMM), the intersection can produce a range narrower than
> reality. The verifier then believes it knows the register's exact value,
> while at runtime the actual hook return value is loaded, creating a
> verifier/runtime mismatch that can be used to bypass BPF memory safety
> checks.
> 
> The else branch already calls mark_reg_unknown() to reset register state
> before any narrowing. Apply the same reset in the is_retval path so
> stale bounds are cleared before __mark_reg_s32_range() intersects.

hi,
you need to specify the bpf tree in the subject "[PATCH bpf] ..."

jirka


> 
> Fixes: 5d99e198be27 ("bpf, lsm: Add check for BPF LSM return value")
> Cc: stable@vger.kernel.org
> Signed-off-by: Tristan Madani <tristan@talencesecurity.com>
> ---
>  kernel/bpf/verifier.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 54c6953a8b84..7e30dddc7721 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -7532,6 +7532,7 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
>  			 */
>  			if (info.reg_type == SCALAR_VALUE) {
>  				if (info.is_retval && get_func_retval_range(env->prog, &range)) {
> +					mark_reg_unknown(env, regs, value_regno);
>  					err = __mark_reg_s32_range(env, regs, value_regno,
>  								   range.minval, range.maxval);
>  					if (err)
> -- 
> 2.47.3
> 
>