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[PATCH] net: filter: make JITs zero A for SKF_AD_ALU_XOR_X

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  [PATCH] net: filter: make JITs zero A for SKF_AD_ALU_XOR_X Rabin Vincent <rabin@rab.in> - 2016-01-05 16:30 +0100
    Re: [PATCH] net: filter: make JITs zero A for SKF_AD_ALU_XOR_X Eric Dumazet <eric.dumazet@gmail.com> - 2016-01-05 17:10 +0100
      Re: [PATCH] net: filter: make JITs zero A for SKF_AD_ALU_XOR_X Rabin Vincent <rabin@rab.in> - 2016-01-05 17:10 +0100
        Re: [PATCH] net: filter: make JITs zero A for SKF_AD_ALU_XOR_X Daniel Borkmann <daniel@iogearbox.net> - 2016-01-05 17:40 +0100
    Re: [PATCH] net: filter: make JITs zero A for SKF_AD_ALU_XOR_X Daniel Borkmann <daniel@iogearbox.net> - 2016-01-05 17:40 +0100
      Re: [PATCH] net: filter: make JITs zero A for SKF_AD_ALU_XOR_X Alexei Starovoitov <alexei.starovoitov@gmail.com> - 2016-01-05 18:50 +0100
    Re: [PATCH] net: filter: make JITs zero A for SKF_AD_ALU_XOR_X David Miller <davem@davemloft.net> - 2016-01-06 06:50 +0100

#1301605 — [PATCH] net: filter: make JITs zero A for SKF_AD_ALU_XOR_X

The SKF_AD_ALU_XOR_X ancillary is not like the other ancillary data
instructions since it XORs A with X while all the others replace A with
some loaded value.  All the BPF JITs fail to clear A if this is used as
the first instruction in a filter.  This was found using american fuzzy
lop.

Add a helper to determine if A needs to be cleared given the first
instruction in a filter, and use this in the JITs.  Except for ARM, the
rest have only been compile-tested.

Fixes: 3480593131e0 ("net: filter: get rid of BPF_S_* enum")
Signed-off-by: Rabin Vincent <rabin@rab.in>
---
 arch/arm/net/bpf_jit_32.c       | 16 +---------------
 arch/mips/net/bpf_jit.c         | 16 +---------------
 arch/powerpc/net/bpf_jit_comp.c | 13 ++-----------
 arch/sparc/net/bpf_jit_comp.c   | 17 ++---------------
 include/linux/filter.h          | 19 +++++++++++++++++++
 5 files changed, 25 insertions(+), 56 deletions(-)

diff --git a/arch/arm/net/bpf_jit_32.c b/arch/arm/net/bpf_jit_32.c
index 591f9db3bf40..e153eb065fe4 100644
--- a/arch/arm/net/bpf_jit_32.c
+++ b/arch/arm/net/bpf_jit_32.c
@@ -187,19 +187,6 @@ static inline int mem_words_used(struct jit_ctx *ctx)
 	return fls(ctx->seen & SEEN_MEM);
 }
 
-static inline bool is_load_to_a(u16 inst)
-{
-	switch (inst) {
-	case BPF_LD | BPF_W | BPF_LEN:
-	case BPF_LD | BPF_W | BPF_ABS:
-	case BPF_LD | BPF_H | BPF_ABS:
-	case BPF_LD | BPF_B | BPF_ABS:
-		return true;
-	default:
-		return false;
-	}
-}
-
 static void jit_fill_hole(void *area, unsigned int size)
 {
 	u32 *ptr;
@@ -211,7 +198,6 @@ static void jit_fill_hole(void *area, unsigned int size)
 static void build_prologue(struct jit_ctx *ctx)
 {
 	u16 reg_set = saved_regs(ctx);
-	u16 first_inst = ctx->skf->insns[0].code;
 	u16 off;
 
 #ifdef CONFIG_FRAME_POINTER
@@ -241,7 +227,7 @@ static void build_prologue(struct jit_ctx *ctx)
 		emit(ARM_MOV_I(r_X, 0), ctx);
 
 	/* do not leak kernel data to userspace */
-	if ((first_inst != (BPF_RET | BPF_K)) && !(is_load_to_a(first_inst)))
+	if (bpf_needs_clear_a(&ctx->skf->insns[0]))
 		emit(ARM_MOV_I(r_A, 0), ctx);
 
 	/* stack space for the BPF_MEM words */
diff --git a/arch/mips/net/bpf_jit.c b/arch/mips/net/bpf_jit.c
index 77cb27309db2..1a8c96035716 100644
--- a/arch/mips/net/bpf_jit.c
+++ b/arch/mips/net/bpf_jit.c
@@ -521,19 +521,6 @@ static inline u16 align_sp(unsigned int num)
 	return num;
 }
 
-static bool is_load_to_a(u16 inst)
-{
-	switch (inst) {
-	case BPF_LD | BPF_W | BPF_LEN:
-	case BPF_LD | BPF_W | BPF_ABS:
-	case BPF_LD | BPF_H | BPF_ABS:
-	case BPF_LD | BPF_B | BPF_ABS:
-		return true;
-	default:
-		return false;
-	}
-}
-
 static void save_bpf_jit_regs(struct jit_ctx *ctx, unsigned offset)
 {
 	int i = 0, real_off = 0;
@@ -614,7 +601,6 @@ static unsigned int get_stack_depth(struct jit_ctx *ctx)
 
 static void build_prologue(struct jit_ctx *ctx)
 {
-	u16 first_inst = ctx->skf->insns[0].code;
 	int sp_off;
 
 	/* Calculate the total offset for the stack pointer */
@@ -641,7 +627,7 @@ static void build_prologue(struct jit_ctx *ctx)
 		emit_jit_reg_move(r_X, r_zero, ctx);
 
 	/* Do not leak kernel data to userspace */
-	if ((first_inst != (BPF_RET | BPF_K)) && !(is_load_to_a(first_inst)))
+	if (bpf_needs_clear_a(&ctx->skf->insns[0]))
 		emit_jit_reg_move(r_A, r_zero, ctx);
 }
 
diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
index 04782164ee67..2d66a8446198 100644
--- a/arch/powerpc/net/bpf_jit_comp.c
+++ b/arch/powerpc/net/bpf_jit_comp.c
@@ -78,18 +78,9 @@ static void bpf_jit_build_prologue(struct bpf_prog *fp, u32 *image,
 		PPC_LI(r_X, 0);
 	}
 
-	switch (filter[0].code) {
-	case BPF_RET | BPF_K:
-	case BPF_LD | BPF_W | BPF_LEN:
-	case BPF_LD | BPF_W | BPF_ABS:
-	case BPF_LD | BPF_H | BPF_ABS:
-	case BPF_LD | BPF_B | BPF_ABS:
-		/* first instruction sets A register (or is RET 'constant') */
-		break;
-	default:
-		/* make sure we dont leak kernel information to user */
+	/* make sure we dont leak kernel information to user */
+	if (bpf_needs_clear_a(&filter[0]))
 		PPC_LI(r_A, 0);
-	}
 }
 
 static void bpf_jit_build_epilogue(u32 *image, struct codegen_context *ctx)
diff --git a/arch/sparc/net/bpf_jit_comp.c b/arch/sparc/net/bpf_jit_comp.c
index 22564f5f2364..3e6e05a7c4c2 100644
--- a/arch/sparc/net/bpf_jit_comp.c
+++ b/arch/sparc/net/bpf_jit_comp.c
@@ -420,22 +420,9 @@ void bpf_jit_compile(struct bpf_prog *fp)
 		}
 		emit_reg_move(O7, r_saved_O7);
 
-		switch (filter[0].code) {
-		case BPF_RET | BPF_K:
-		case BPF_LD | BPF_W | BPF_LEN:
-		case BPF_LD | BPF_W | BPF_ABS:
-		case BPF_LD | BPF_H | BPF_ABS:
-		case BPF_LD | BPF_B | BPF_ABS:
-			/* The first instruction sets the A register (or is
-			 * a "RET 'constant'")
-			 */
-			break;
-		default:
-			/* Make sure we dont leak kernel information to the
-			 * user.
-			 */
+		/* Make sure we dont leak kernel information to the user. */
+		if (bpf_needs_clear_a(&filter[0]))
 			emit_clear(r_A); /* A = 0 */
-		}
 
 		for (i = 0; i < flen; i++) {
 			unsigned int K = filter[i].k;
diff --git a/include/linux/filter.h b/include/linux/filter.h
index 4165e9ac9e36..5972ffe5719a 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -493,6 +493,25 @@ static inline void bpf_jit_free(struct bpf_prog *fp)
 
 #define BPF_ANC		BIT(15)
 
+static inline bool bpf_needs_clear_a(const struct sock_filter *first)
+{
+	switch (first->code) {
+	case BPF_RET | BPF_K:
+	case BPF_LD | BPF_W | BPF_LEN:
+		return false;
+
+	case BPF_LD | BPF_W | BPF_ABS:
+	case BPF_LD | BPF_H | BPF_ABS:
+	case BPF_LD | BPF_B | BPF_ABS:
+		if (first->k == SKF_AD_OFF + SKF_AD_ALU_XOR_X)
+			return true;
+		return false;
+
+	default:
+		return true;
+	}
+}
+
 static inline u16 bpf_anc_helper(const struct sock_filter *ftest)
 {
 	BUG_ON(ftest->code & BPF_ANC);
-- 
2.6.4

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#1301652

On Tue, 2016-01-05 at 16:23 +0100, Rabin Vincent wrote:
> The SKF_AD_ALU_XOR_X ancillary is not like the other ancillary data
> instructions since it XORs A with X while all the others replace A with
> some loaded value.  All the BPF JITs fail to clear A if this is used as
> the first instruction in a filter.

Is x86_64 part of this 'All' subset ? ;)

>   This was found using american fuzzy
> lop.
> 
> Add a helper to determine if A needs to be cleared given the first
> instruction in a filter, and use this in the JITs.  Except for ARM, the
> rest have only been compile-tested.
> 
> Fixes: 3480593131e0 ("net: filter: get rid of BPF_S_* enum")
> Signed-off-by: Rabin Vincent <rabin@rab.in>
> ---


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#1301655

On Tue, Jan 05, 2016 at 08:00:45AM -0800, Eric Dumazet wrote:
> On Tue, 2016-01-05 at 16:23 +0100, Rabin Vincent wrote:
> > The SKF_AD_ALU_XOR_X ancillary is not like the other ancillary data
> > instructions since it XORs A with X while all the others replace A with
> > some loaded value.  All the BPF JITs fail to clear A if this is used as
> > the first instruction in a filter.
> 
> Is x86_64 part of this 'All' subset ? ;)

No, because it's an eBPF JIT.
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#1301689

On 01/05/2016 05:03 PM, Rabin Vincent wrote:
> On Tue, Jan 05, 2016 at 08:00:45AM -0800, Eric Dumazet wrote:
>> On Tue, 2016-01-05 at 16:23 +0100, Rabin Vincent wrote:
>>> The SKF_AD_ALU_XOR_X ancillary is not like the other ancillary data
>>> instructions since it XORs A with X while all the others replace A with
>>> some loaded value.  All the BPF JITs fail to clear A if this is used as
>>> the first instruction in a filter.
>>
>> Is x86_64 part of this 'All' subset ? ;)
>
> No, because it's an eBPF JIT.

Correct, filter conversion to eBPF clears it already.
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#1301683

On 01/05/2016 04:23 PM, Rabin Vincent wrote:
> The SKF_AD_ALU_XOR_X ancillary is not like the other ancillary data
> instructions since it XORs A with X while all the others replace A with
> some loaded value.  All the BPF JITs fail to clear A if this is used as
> the first instruction in a filter.  This was found using american fuzzy
> lop.
>
> Add a helper to determine if A needs to be cleared given the first
> instruction in a filter, and use this in the JITs.  Except for ARM, the
> rest have only been compile-tested.
>
> Fixes: 3480593131e0 ("net: filter: get rid of BPF_S_* enum")
> Signed-off-by: Rabin Vincent <rabin@rab.in>

Excellent catch, thanks a lot! The fix looks good to me and should
go to -net tree.

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

If you're interested, feel free to add a small test case for the
SKF_AD_ALU_XOR_X issue to lib/test_bpf.c for -net-next tree. Thanks!
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#1301745

On Tue, Jan 05, 2016 at 05:36:47PM +0100, Daniel Borkmann wrote:
> On 01/05/2016 04:23 PM, Rabin Vincent wrote:
> >The SKF_AD_ALU_XOR_X ancillary is not like the other ancillary data
> >instructions since it XORs A with X while all the others replace A with
> >some loaded value.  All the BPF JITs fail to clear A if this is used as
> >the first instruction in a filter.  This was found using american fuzzy
> >lop.
> >
> >Add a helper to determine if A needs to be cleared given the first
> >instruction in a filter, and use this in the JITs.  Except for ARM, the
> >rest have only been compile-tested.
> >
> >Fixes: 3480593131e0 ("net: filter: get rid of BPF_S_* enum")
> >Signed-off-by: Rabin Vincent <rabin@rab.in>
> 
> Excellent catch, thanks a lot! The fix looks good to me and should
> go to -net tree.
> 
> Acked-by: Daniel Borkmann <daniel@iogearbox.net>

good catch indeed.
Classic bpf jits didn't have much love. Great to see this work.

Acked-by: Alexei Starovoitov <ast@kernel.org>

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#1302406

From: Rabin Vincent <rabin@rab.in>
Date: Tue,  5 Jan 2016 16:23:07 +0100

> The SKF_AD_ALU_XOR_X ancillary is not like the other ancillary data
> instructions since it XORs A with X while all the others replace A with
> some loaded value.  All the BPF JITs fail to clear A if this is used as
> the first instruction in a filter.  This was found using american fuzzy
> lop.
> 
> Add a helper to determine if A needs to be cleared given the first
> instruction in a filter, and use this in the JITs.  Except for ARM, the
> rest have only been compile-tested.
> 
> Fixes: 3480593131e0 ("net: filter: get rid of BPF_S_* enum")
> Signed-off-by: Rabin Vincent <rabin@rab.in>

Applied and queued up for -stable, thanks!
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