CVE-2016-2383 incorrect branch fixups for eBPG allow arbitrary reads (rhbz 1308452 1308453)

bpf-fix-branch-offset-adjustment-on-backjumps-after-.patch

@@ -0,0 +1,92 @@
+From a1b14d27ed0965838350f1377ff97c93ee383492 Mon Sep 17 00:00:00 2001
+From: Daniel Borkmann <daniel@iogearbox.net>
+Date: Wed, 10 Feb 2016 16:47:11 +0100
+Subject: [PATCH] bpf: fix branch offset adjustment on backjumps after patching
+ ctx expansion
+
+When ctx access is used, the kernel often needs to expand/rewrite
+instructions, so after that patching, branch offsets have to be
+adjusted for both forward and backward jumps in the new eBPF program,
+but for backward jumps it fails to account the delta. Meaning, for
+example, if the expansion happens exactly on the insn that sits at
+the jump target, it doesn't fix up the back jump offset.
+
+Analysis on what the check in adjust_branches() is currently doing:
+
+  /* adjust offset of jmps if necessary */
+  if (i < pos && i + insn->off + 1 > pos)
+    insn->off += delta;
+  else if (i > pos && i + insn->off + 1 < pos)
+    insn->off -= delta;
+
+First condition (forward jumps):
+
+  Before:                         After:
+
+  insns[0]                        insns[0]
+  insns[1] <--- i/insn            insns[1] <--- i/insn
+  insns[2] <--- pos               insns[P] <--- pos
+  insns[3]                        insns[P]  `------| delta
+  insns[4] <--- target_X          insns[P]   `-----|
+  insns[5]                        insns[3]
+                                  insns[4] <--- target_X
+                                  insns[5]
+
+First case is if we cross pos-boundary and the jump instruction was
+before pos. This is handeled correctly. I.e. if i == pos, then this
+would mean our jump that we currently check was the patchlet itself
+that we just injected. Since such patchlets are self-contained and
+have no awareness of any insns before or after the patched one, the
+delta is correctly not adjusted. Also, for the second condition in
+case of i + insn->off + 1 == pos, means we jump to that newly patched
+instruction, so no offset adjustment are needed. That part is correct.
+
+Second condition (backward jumps):
+
+  Before:                         After:
+
+  insns[0]                        insns[0]
+  insns[1] <--- target_X          insns[1] <--- target_X
+  insns[2] <--- pos <-- target_Y  insns[P] <--- pos <-- target_Y
+  insns[3]                        insns[P]  `------| delta
+  insns[4] <--- i/insn            insns[P]   `-----|
+  insns[5]                        insns[3]
+                                  insns[4] <--- i/insn
+                                  insns[5]
+
+Second interesting case is where we cross pos-boundary and the jump
+instruction was after pos. Backward jump with i == pos would be
+impossible and pose a bug somewhere in the patchlet, so the first
+condition checking i > pos is okay only by itself. However, i +
+insn->off + 1 < pos does not always work as intended to trigger the
+adjustment. It works when jump targets would be far off where the
+delta wouldn't matter. But, for example, where the fixed insn->off
+before pointed to pos (target_Y), it now points to pos + delta, so
+that additional room needs to be taken into account for the check.
+This means that i) both tests here need to be adjusted into pos + delta,
+and ii) for the second condition, the test needs to be <= as pos
+itself can be a target in the backjump, too.
+
+Fixes: 9bac3d6d548e ("bpf: allow extended BPF programs access skb fields")
+Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
+Signed-off-by: David S. Miller <davem@davemloft.net>
+---
+ kernel/bpf/verifier.c | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
+index d1d3e8f57de9..2e7f7ab739e4 100644
+--- a/kernel/bpf/verifier.c
++++ b/kernel/bpf/verifier.c
+@@ -2082,7 +2082,7 @@ static void adjust_branches(struct bpf_prog *prog, int pos, int delta)
+ 		/* adjust offset of jmps if necessary */
+ 		if (i < pos && i + insn->off + 1 > pos)
+ 			insn->off += delta;
+-		else if (i > pos && i + insn->off + 1 < pos)
++		else if (i > pos + delta && i + insn->off + 1 <= pos + delta)
+ 			insn->off -= delta;
+ 	}
+ }
+-- 
+2.5.0
+

kernel.spec

@@ -634,6 +634,9 @@ Patch648: fs-hugetlbfs-inode.c-fix-bugs-in-hugetlb_vmtruncate_.patch
 #CVE-2016-2384 rhbz 1308444 1308445
 Patch649: ALSA-usb-audio-avoid-freeing-umidi-object-twice.patch
 
+#CVE-2016-2383 rhbz 1308452 1308453
+Patch650: bpf-fix-branch-offset-adjustment-on-backjumps-after-.patch
+
 # END OF PATCH DEFINITIONS
 
 %endif
@@ -2078,6 +2081,7 @@ fi
 # 
 %changelog
 * Mon Feb 15 2016 Josh Boyer <jwboyer@fedoraproject.org>
+- CVE-2016-2383 incorrect branch fixups for eBPG allow arbitrary reads (rhbz 1308452 1308453)
 - CVE-2016-2384 double free in usb-audio from invalid USB descriptor (rhbz 1308444 1308445)
 
 * Fri Feb 12 2016 Laura Abbott <labbott@fedoraproject.org>