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([2001:41d0:303:db6b::]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-49230a4ff67sm153691355e9.6.2026.06.17.05.08.15 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 17 Jun 2026 05:08:16 -0700 (PDT) From: Tristan Madani To: Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko Cc: Xu Kuohai , Eduard Zingerman , bpf@vger.kernel.org, stable@vger.kernel.org, Tristan Madani Subject: [PATCH] bpf: Reset register bounds before narrowing retval range in check_mem_access() Date: Wed, 17 Jun 2026 12:08:15 +0000 Message-ID: <20260617120815.3910671-1-tristmd@gmail.com> X-Mailer: git-send-email 2.47.3 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Tristan Madani 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. Fixes: 5d99e198be27 ("bpf, lsm: Add check for BPF LSM return value") Cc: stable@vger.kernel.org Signed-off-by: Tristan Madani --- 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