From: Piotr Krysiuk <piotras@gmail.com>

Date: Mon, 5 Apr 2021 22:52:15 +0100

Subject: bpf, x86: Validate computation of branch displacements for x86-64

Patch-mainline: v5.12-rc7

Git-commit: e4d4d456436bfb2fe412ee2cd489f7658449b098

References: bsc#1184391 CVE-2021-29154

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@gmail.com>

Reviewed-by: Daniel Borkmann <daniel@iogearbox.net>

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

Acked-by: Gary Lin <glin@suse.com>

---

 arch/x86/net/bpf_jit_comp.c |   11 ++++++++++-

 1 file changed, 10 insertions(+), 1 deletion(-)

--- a/arch/x86/net/bpf_jit_comp.c

+++ b/arch/x86/net/bpf_jit_comp.c

@@ -1394,7 +1394,16 @@ emit_jmp:

 		}

 		if (image) {

-			if (unlikely(proglen + ilen > oldproglen)) {

+			/*

+			 * 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;

 			}