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

Message ID 20210323202826.159294-3-cascardo@canonical.com
State New
Headers show
Series [SRU,Focal/Groovy,1/3] bpf: Simplify alu_limit masking for pointer arithmetic | expand

Commit Message

Thadeu Lima de Souza Cascardo March 23, 2021, 8:28 p.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>
(cherry picked from commit 1b1597e64e1a610c7a96710fc4717158e98a08b3 net.git)
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 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;
 	}