[PATCH net] net: filter: x86: fix JIT address randomization

[Alexei Starovoitov <ast@plumgrid.com>]

bpf_alloc_binary() adds 128 bytes of room to JITed program image
and rounds it up to the nearest page size. If image size is close
to page size (like 4000), it is rounded to two pages:
round_up(4000 + 4 + 128) == 8192
then 'hole' is computed as 8192 - (4000 + 4) = 4188
If prandom_u32() % hole selects a number >= 4096, then kernel will crash
during bpf_jit_free():

kernel BUG at arch/x86/mm/pageattr.c:887!
Call Trace:
 [<ffffffff81037285>] change_page_attr_set_clr+0x135/0x460
 [<ffffffff81694cc0>] ? _raw_spin_unlock_irq+0x30/0x50
 [<ffffffff810378ff>] set_memory_rw+0x2f/0x40
 [<ffffffffa01a0d8d>] bpf_jit_free_deferred+0x2d/0x60
 [<ffffffff8106bf98>] process_one_work+0x1d8/0x6a0
 [<ffffffff8106bf38>] ? process_one_work+0x178/0x6a0
 [<ffffffff8106c90c>] worker_thread+0x11c/0x370

since bpf_jit_free() does:
  unsigned long addr = (unsigned long)fp->bpf_func & PAGE_MASK;
  struct bpf_binary_header *header = (void *)addr;
to compute start address of 'bpf_binary_header'
and header->pages will pass junk to:
  set_memory_rw(addr, header->pages);

Fix it by picking image offset always out of the first page.

While at it make the offset to be within first half of the page,
so there is some room for CPU to run before it hits page miss.

Fixes: 314beb9bcabfd ("x86: bpf_jit_comp: secure bpf jit against spraying attacks")
Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
---

s390 commit aa2d2c73c21f ("s390/bpf,jit: address randomize and write protect jit code")
seems to have the same problem

 arch/x86/net/bpf_jit_comp.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index dc01773..c6ab7a0 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -171,7 +171,7 @@ static struct bpf_binary_header *bpf_alloc_binary(unsigned int proglen,
 	memset(header, 0xcc, sz); /* fill whole space with int3 instructions */
 
 	header->pages = sz / PAGE_SIZE;
-	hole = sz - (proglen + sizeof(*header));
+	hole = min(sz - (proglen + sizeof(*header)), PAGE_SIZE / 2);
 
 	/* insert a random number of int3 instructions before BPF code */
 	*image_ptr = &header->image[prandom_u32() % hole];
-- 
1.7.9.5