[SRU Focal/Groovy 2/3] bpf: Add sanity check for upper ptr_limit

[Thadeu Lima de Souza Cascardo]

From: Piotr Krysiuk <piotras at 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 at gmail.com>
Co-developed-by: Daniel Borkmann <daniel at iogearbox.net>
Signed-off-by: Daniel Borkmann <daniel at iogearbox.net>
Acked-by: Alexei Starovoitov <ast at kernel.org>
(cherry picked from commit 1b1597e64e1a610c7a96710fc4717158e98a08b3 net.git)
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
---
 kernel/bpf/verifier.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index aaec49eb4f87..9d413f7dffac 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4914,10 +4914,14 @@ 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;
 		/* Indirect variable offset stack access is prohibited in
 		 * unprivileged mode so it's not handled here.
 		 */
@@ -4926,15 +4930,16 @@ static int retrieve_ptr_limit(const struct bpf_reg_state *ptr_reg,
 			*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;
 	}
-- 
2.27.0