[3.19.y-ckt stable] Patch "ARM: net: delegate filter to kernel interpreter when imm_offset() return value can't fit into 12bits." has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    ARM: net: delegate filter to kernel interpreter when imm_offset() return value can't fit into 12bits.

to the linux-3.19.y-queue branch of the 3.19.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.19.y-queue

This patch is scheduled to be released in version 3.19.8-ckt2.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.19.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From 1f5d30ad54e3312875cb98813314a04d8f32f68c Mon Sep 17 00:00:00 2001
From: Nicolas Schichan <nschichan at freebox.fr>
Date: Thu, 7 May 2015 17:14:21 +0200
Subject: ARM: net: delegate filter to kernel interpreter when imm_offset()
 return value can't fit into 12bits.

commit 0b59d8806a31bb0267b3a461e8fef20c727bdbf6 upstream.

The ARM JIT code emits "ldr rX, [pc, #offset]" to access the literal
pool. #offset maximum value is 4095 and if the generated code is too
large, the #offset value can overflow and not point to the expected
slot in the literal pool. Additionally, when overflow occurs, bits of
the overflow can end up changing the destination register of the ldr
instruction.

Fix that by detecting the overflow in imm_offset() and setting a flag
that is checked for each BPF instructions converted in
build_body(). As of now it can only be detected in the second pass. As
a result the second build_body() call can now fail, so add the
corresponding cleanup code in that case.

Using multiple literal pools in the JITed code is going to require
lots of intrusive changes to the JIT code (which would better be done
as a feature instead of fix), just delegating to the kernel BPF
interpreter in that case is a more straight forward, minimal fix and
easy to backport.

Fixes: ddecdfcea0ae ("ARM: 7259/3: net: JIT compiler for packet filters")
Signed-off-by: Nicolas Schichan <nschichan at freebox.fr>
Acked-by: Daniel Borkmann <daniel at iogearbox.net>
Signed-off-by: David S. Miller <davem at davemloft.net>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 arch/arm/net/bpf_jit_32.c | 27 ++++++++++++++++++++++++++-
 1 file changed, 26 insertions(+), 1 deletion(-)

diff --git a/arch/arm/net/bpf_jit_32.c b/arch/arm/net/bpf_jit_32.c
index f412b53..e0e2358 100644
--- a/arch/arm/net/bpf_jit_32.c
+++ b/arch/arm/net/bpf_jit_32.c
@@ -54,6 +54,7 @@
 #define SEEN_DATA		(1 << (BPF_MEMWORDS + 3))

 #define FLAG_NEED_X_RESET	(1 << 0)
+#define FLAG_IMM_OVERFLOW	(1 << 1)

 struct jit_ctx {
 	const struct bpf_prog *skf;
@@ -293,6 +294,15 @@ static u16 imm_offset(u32 k, struct jit_ctx *ctx)
 	/* PC in ARM mode == address of the instruction + 8 */
 	imm = offset - (8 + ctx->idx * 4);

+	if (imm & ~0xfff) {
+		/*
+		 * literal pool is too far, signal it into flags. we
+		 * can only detect it on the second pass unfortunately.
+		 */
+		ctx->flags |= FLAG_IMM_OVERFLOW;
+		return 0;
+	}
+
 	return imm;
 }

@@ -866,6 +876,14 @@ b_epilogue:
 		default:
 			return -1;
 		}
+
+		if (ctx->flags & FLAG_IMM_OVERFLOW)
+			/*
+			 * this instruction generated an overflow when
+			 * trying to access the literal pool, so
+			 * delegate this filter to the kernel interpreter.
+			 */
+			return -1;
 	}

 	/* compute offsets only during the first pass */
@@ -928,7 +946,14 @@ void bpf_jit_compile(struct bpf_prog *fp)
 	ctx.idx = 0;

 	build_prologue(&ctx);
-	build_body(&ctx);
+	if (build_body(&ctx) < 0) {
+#if __LINUX_ARM_ARCH__ < 7
+		if (ctx.imm_count)
+			kfree(ctx.imms);
+#endif
+		bpf_jit_binary_free(header);
+		goto out;
+	}
 	build_epilogue(&ctx);

 	flush_icache_range((u32)ctx.target, (u32)(ctx.target + ctx.idx));
--
1.9.1