[SRU Xenial 1/1] UBUNTU: SAUCE: bpf, x86: Validate computation of branch displacements for x86-64

[Thadeu Lima de Souza Cascardo]

From: Piotr Krysiuk <piotras at gmail.com>

The branch displacement logic in the BPF JIT compilers for x86 assumes
that, for any generated branch instruction, the distance cannot
increase between optimization passes.

But this assumption can be violated due to how the distances are
computed. Specifically, whenever a backward branch is processed in
do_jit(), the distance is computed by subtracting the positions in the
machine code from different optimization passes. This is because part
of addrs[] is already updated for the current optimization pass, before
the branch instruction is visited.

And so the optimizer can expand blocks of machine code in some cases.

This can confuse the optimizer logic, where it assumes that a fixed
point has been reached for all machine code blocks once the total
program size stops changing. And then the JIT compiler can output
abnormal machine code containing incorrect branch displacements.

To mitigate this issue, we assert that a fixed point is reached while
populating the output image. This rejects any problematic programs.
The issue affects both x86-32 and x86-64. We mitigate separately to
ease backporting.

Signed-off-by: Piotr Krysiuk <piotras at gmail.com>
Reviewed-by: Daniel Borkmann <daniel at iogearbox.net>
Signed-off-by: Daniel Borkmann <daniel at iogearbox.net>
(backported from commit e4d4d456436bfb2fe412ee2cd489f7658449b098)
 git://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git)
[cascardo: conflict on message, kept new one, so users can find more
 references about it]
CVE-2021-29154
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
---
 arch/x86/net/bpf_jit_comp.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 3167233fe6c1..177ef46c05da 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -1061,8 +1061,17 @@ common_load:
 		}
 
 		if (image) {
-			if (unlikely(proglen + ilen > oldproglen)) {
-				pr_err("bpf_jit_compile fatal error\n");
+			/*
+			 * When populating the image, assert that:
+			 *
+			 *  i) We do not write beyond the allocated space, and
+			 * ii) addrs[i] did not change from the prior run, in order
+			 *     to validate assumptions made for computing branch
+			 *     displacements.
+			 */
+			if (unlikely(proglen + ilen > oldproglen ||
+				     proglen + ilen != addrs[i])) {
+				pr_err("bpf_jit: fatal error\n");
 				return -EFAULT;
 			}
 			memcpy(image + proglen, temp, ilen);
-- 
2.27.0