bpf/verifier: fix bounds calculation on BPF_RSH

From: Edward Cree <ecree@solarflare.com>

[ Upstream commit 4374f256ce8182019353c0c639bb8d0695b4c941 ]

Incorrect signed bounds were being computed.
If the old upper signed bound was positive and the old lower signed bound was
negative, this could cause the new upper signed bound to be too low,
leading to security issues.

Fixes: b03c9f9fdc37 ("bpf/verifier: track signed and unsigned min/max values")
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Edward Cree <ecree@solarflare.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
[jannh@google.com: changed description to reflect bug impact]
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 201d6b965811f246bfce439298ec32ce10845da9..c6ec04f94525558fe5a5ee0728609837f5f7e3d9 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2157,20 +2157,22 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
                        mark_reg_unknown(regs, insn->dst_reg);
                        break;
                }
-               /* BPF_RSH is an unsigned shift, so make the appropriate casts */
-               if (dst_reg->smin_value < 0) {
-                       if (umin_val) {
-                               /* Sign bit will be cleared */
-                               dst_reg->smin_value = 0;
-                       } else {
-                               /* Lost sign bit information */
-                               dst_reg->smin_value = S64_MIN;
-                               dst_reg->smax_value = S64_MAX;
-                       }
-               } else {
-                       dst_reg->smin_value =
-                               (u64)(dst_reg->smin_value) >> umax_val;
-               }
+               /* BPF_RSH is an unsigned shift.  If the value in dst_reg might
+                * be negative, then either:
+                * 1) src_reg might be zero, so the sign bit of the result is
+                *    unknown, so we lose our signed bounds
+                * 2) it's known negative, thus the unsigned bounds capture the
+                *    signed bounds
+                * 3) the signed bounds cross zero, so they tell us nothing
+                *    about the result
+                * If the value in dst_reg is known nonnegative, then again the
+                * unsigned bounts capture the signed bounds.
+                * Thus, in all cases it suffices to blow away our signed bounds
+                * and rely on inferring new ones from the unsigned bounds and
+                * var_off of the result.
+                */
+               dst_reg->smin_value = S64_MIN;
+               dst_reg->smax_value = S64_MAX;
                if (src_known)
                        dst_reg->var_off = tnum_rshift(dst_reg->var_off,
                                                       umin_val);