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[SRU,Bionic,2/3] bpf: Add sanity check for upper ptr_limit

Message ID 20210324004337.170117-3-cascardo@canonical.com
State New
Headers show
Series (LP: #1920995) selftests: bpf verifier fails after sanitize_ptr_alu fixes | expand

Commit Message

Thadeu Lima de Souza Cascardo March 24, 2021, 12:43 a.m. UTC 
From: Piotr Krysiuk <piotras@gmail.com>

BugLink: https://bugs.launchpad.net/bugs/1920995

Given we know the max possible value of ptr_limit at the time of retrieving
the latter, add basic assertions, so that the verifier can bail out if
anything looks odd and reject the program. Nothing triggered this so far,
but it also does not hurt to have these.

Signed-off-by: Piotr Krysiuk <piotras@gmail.com>
Co-developed-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
(backported from commit 1b1597e64e1a610c7a96710fc4717158e98a08b3 net.git)
[cascardo: context adjustment because of missing commit 088ec26d9c2d,
  left introduced comment out to reflect reality]
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
---
 kernel/bpf/verifier.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)
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Patch

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index a9086c4a03b5..05b092605649 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1988,24 +1988,29 @@  static int retrieve_ptr_limit(const struct bpf_reg_state *ptr_reg,
 {
 	bool mask_to_left = (opcode == BPF_ADD &&  off_is_neg) ||
 			    (opcode == BPF_SUB && !off_is_neg);
-	u32 off;
+	u32 off, max;
 
 	switch (ptr_reg->type) {
 	case PTR_TO_STACK:
+		/* Offset 0 is out-of-bounds, but acceptable start for the
+		 * left direction, see BPF_REG_FP.
+		 */
+		max = MAX_BPF_STACK + mask_to_left;
 		off = ptr_reg->off + ptr_reg->var_off.value;
 		if (mask_to_left)
 			*ptr_limit = MAX_BPF_STACK + off;
 		else
 			*ptr_limit = -off - 1;
-		return 0;
+		return *ptr_limit >= max ? -ERANGE : 0;
 	case PTR_TO_MAP_VALUE:
+		max = ptr_reg->map_ptr->value_size;
 		if (mask_to_left) {
 			*ptr_limit = ptr_reg->umax_value + ptr_reg->off;
 		} else {
 			off = ptr_reg->smin_value + ptr_reg->off;
 			*ptr_limit = ptr_reg->map_ptr->value_size - off - 1;
 		}
-		return 0;
+		return *ptr_limit >= max ? -ERANGE : 0;
 	default:
 		return -EINVAL;
 	}