[bpf,1/5] bpf, s390x: do not reload skb pointers in non-skb context

Message ID 20171214200727.22230-2-daniel@iogearbox.net
State Accepted
Delegated to: BPF Maintainers
Headers show
Series
  • Couple of BPF JIT fixes
Related show

Commit Message

Daniel Borkmann Dec. 14, 2017, 8:07 p.m.
The assumption of unconditionally reloading skb pointers on
BPF helper calls where bpf_helper_changes_pkt_data() holds
true is wrong. There can be different contexts where the
BPF helper would enforce a reload such as in case of XDP.
Here, we do have a struct xdp_buff instead of struct sk_buff
as context, thus this will access garbage.

JITs only ever need to deal with cached skb pointer reload
when ld_abs/ind was seen, therefore guard the reload behind
SEEN_SKB only. Tested on s390x.

Fixes: 9db7f2b81880 ("s390/bpf: recache skb->data/hlen for skb_vlan_push/pop")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Cc: Michael Holzheu <holzheu@linux.vnet.ibm.com>
---
 arch/s390/net/bpf_jit_comp.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

Comments

Michael Holzheu Jan. 9, 2018, 2:20 p.m. | #1
Am Thu, 14 Dec 2017 21:07:23 +0100
schrieb Daniel Borkmann <daniel@iogearbox.net>:

> The assumption of unconditionally reloading skb pointers on
> BPF helper calls where bpf_helper_changes_pkt_data() holds
> true is wrong. There can be different contexts where the
> BPF helper would enforce a reload such as in case of XDP.
> Here, we do have a struct xdp_buff instead of struct sk_buff
> as context, thus this will access garbage.
> 
> JITs only ever need to deal with cached skb pointer reload
> when ld_abs/ind was seen, therefore guard the reload behind
> SEEN_SKB only. Tested on s390x.

Hello Daniel,

Sorry for the late answer - I have been on vacation up to now.
Thanks for fixing / testing this for s390x.

Michael

Patch

diff --git a/arch/s390/net/bpf_jit_comp.c b/arch/s390/net/bpf_jit_comp.c
index e81c168..9557d8b 100644
--- a/arch/s390/net/bpf_jit_comp.c
+++ b/arch/s390/net/bpf_jit_comp.c
@@ -55,8 +55,7 @@  struct bpf_jit {
 #define SEEN_LITERAL	8	/* code uses literals */
 #define SEEN_FUNC	16	/* calls C functions */
 #define SEEN_TAIL_CALL	32	/* code uses tail calls */
-#define SEEN_SKB_CHANGE	64	/* code changes skb data */
-#define SEEN_REG_AX	128	/* code uses constant blinding */
+#define SEEN_REG_AX	64	/* code uses constant blinding */
 #define SEEN_STACK	(SEEN_FUNC | SEEN_MEM | SEEN_SKB)
 
 /*
@@ -448,12 +447,12 @@  static void bpf_jit_prologue(struct bpf_jit *jit, u32 stack_depth)
 			EMIT6_DISP_LH(0xe3000000, 0x0024, REG_W1, REG_0,
 				      REG_15, 152);
 	}
-	if (jit->seen & SEEN_SKB)
+	if (jit->seen & SEEN_SKB) {
 		emit_load_skb_data_hlen(jit);
-	if (jit->seen & SEEN_SKB_CHANGE)
 		/* stg %b1,ST_OFF_SKBP(%r0,%r15) */
 		EMIT6_DISP_LH(0xe3000000, 0x0024, BPF_REG_1, REG_0, REG_15,
 			      STK_OFF_SKBP);
+	}
 }
 
 /*
@@ -983,8 +982,8 @@  static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp, int i
 		EMIT2(0x0d00, REG_14, REG_W1);
 		/* lgr %b0,%r2: load return value into %b0 */
 		EMIT4(0xb9040000, BPF_REG_0, REG_2);
-		if (bpf_helper_changes_pkt_data((void *)func)) {
-			jit->seen |= SEEN_SKB_CHANGE;
+		if ((jit->seen & SEEN_SKB) &&
+		    bpf_helper_changes_pkt_data((void *)func)) {
 			/* lg %b1,ST_OFF_SKBP(%r15) */
 			EMIT6_DISP_LH(0xe3000000, 0x0004, BPF_REG_1, REG_0,
 				      REG_15, STK_OFF_SKBP);