[PATCH 4.18 022/123] bpf, arm32: fix stack var offset in jit

[Greg Kroah-Hartman]

4.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Daniel Borkmann <daniel@xxxxxxxxxxxxx>

Commit 38ca93060163 ("bpf, arm32: save 4 bytes of unneeded stack
space") messed up STACK_VAR() by 4 bytes presuming it was related
to skb scratch buffer space, but it clearly isn't as this refers
to the top word in stack, therefore restore it. This fixes a NULL
pointer dereference seen during bootup when JIT is enabled and BPF
program run in sk_filter_trim_cap() triggered by systemd-udevd.

JIT rework in 1c35ba122d4a ("ARM: net: bpf: use negative numbers
for stacked registers") and 96cced4e774a ("ARM: net: bpf: access
eBPF scratch space using ARM FP register") removed the affected
parts, so only needed in 4.18 stable.

Fixes: 38ca93060163 ("bpf, arm32: save 4 bytes of unneeded stack space")
Reported-by: Peter Robinson <pbrobinson@xxxxxxxxx>
Reported-by: Marc Haber <mh+netdev@xxxxxxxxxxxx>
Tested-by: Stefan Wahren <stefan.wahren@xxxxxxxx>
Tested-by: Peter Robinson <pbrobinson@xxxxxxxxx>
Cc: Russell King <rmk+kernel@xxxxxxxxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Acked-by: Alexei Starovoitov <ast@xxxxxxxxxx>
---
 arch/arm/net/bpf_jit_32.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/arm/net/bpf_jit_32.c
+++ b/arch/arm/net/bpf_jit_32.c
@@ -238,7 +238,7 @@ static void jit_fill_hole(void *area, un
 #define STACK_SIZE	ALIGN(_STACK_SIZE, STACK_ALIGNMENT)
 
 /* Get the offset of eBPF REGISTERs stored on scratch space. */
-#define STACK_VAR(off) (STACK_SIZE - off)
+#define STACK_VAR(off) (STACK_SIZE - off - 4)
 
 #if __LINUX_ARM_ARCH__ < 7