[Precise][CVE-2015-4700] x86: bpf_jit: fix compilation of large bpf programs

[Kamal Mostafa]

From: Alexei Starovoitov <ast at plumgrid.com>

commit 3f7352bf21f8fd7ba3e2fcef9488756f188e12be upstream.

BugLink: http://bugs.launchpad.net/bugs/1469320

x86 has variable length encoding. x86 JIT compiler is trying
to pick the shortest encoding for given bpf instruction.
While doing so the jump targets are changing, so JIT is doing
multiple passes over the program. Typical program needs 3 passes.
Some very short programs converge with 2 passes. Large programs
may need 4 or 5. But specially crafted bpf programs may hit the
pass limit and if the program converges on the last iteration
the JIT compiler will be producing an image full of 'int 3' insns.
Fix this corner case by doing final iteration over bpf program.

Fixes: 0a14842f5a3c ("net: filter: Just In Time compiler for x86-64")
Reported-by: Daniel Borkmann <daniel at iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast at plumgrid.com>
Tested-by: Daniel Borkmann <daniel at iogearbox.net>
Acked-by: Daniel Borkmann <daniel at iogearbox.net>
Signed-off-by: David S. Miller <davem at davemloft.net>
CVE-2015-4700
[ kamal: backport to 3.8 (Ubuntu Precise): context ]
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 arch/x86/net/bpf_jit_comp.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 5a5b6e4..11e3100 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -150,7 +150,12 @@ void bpf_jit_compile(struct sk_filter *fp)
 	}
 	cleanup_addr = proglen; /* epilogue address */
 
-	for (pass = 0; pass < 10; pass++) {
+	/* JITed image shrinks with every pass and the loop iterates
+	 * until the image stops shrinking. Very large bpf programs
+	 * may converge on the last pass. In such case do one more
+	 * pass to emit the final image
+	 */
+	for (pass = 0; pass < 10 || image; pass++) {
 		u8 seen_or_pass0 = (pass == 0) ? (SEEN_XREG | SEEN_DATAREF | SEEN_MEM) : seen;
 		/* no prologue/epilogue for trivial filters (RET something) */
 		proglen = 0;
-- 
1.9.1