This is a note to let you know that I've just added the patch titled
bpf, x86: Validate computation of branch displacements for x86-32
to the 4.19-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
bpf-x86-validate-computation-of-branch-displacements-for-x86-32.patch
and it can be found in the queue-4.19 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 26f55a59dc65ff77cd1c4b37991e26497fc68049 Mon Sep 17 00:00:00 2001
From: Piotr Krysiuk <piotras@xxxxxxxxx>
Date: Tue, 6 Apr 2021 21:59:39 +0100
Subject: bpf, x86: Validate computation of branch displacements for x86-32
From: Piotr Krysiuk <piotras@xxxxxxxxx>
commit 26f55a59dc65ff77cd1c4b37991e26497fc68049 upstream.
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@xxxxxxxxx>
Reviewed-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
arch/x86/net/bpf_jit_comp32.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)
--- a/arch/x86/net/bpf_jit_comp32.c
+++ b/arch/x86/net/bpf_jit_comp32.c
@@ -2201,7 +2201,16 @@ notyet:
}
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;
}
Patches currently in stable-queue which might be from piotras@xxxxxxxxx are
queue-4.19/bpf-x86-validate-computation-of-branch-displacements-for-x86-64.patch
queue-4.19/bpf-x86-validate-computation-of-branch-displacements-for-x86-32.patch
