[3.13.y.z extended stable] Patch "net: filter: x86: fix JIT address randomization" has been added to staging queue

[Kamal Mostafa]

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

    net: filter: x86: fix JIT address randomization

to the linux-3.13.y-queue branch of the 3.13.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.13.y-queue

This patch is scheduled to be released in version 3.13.11.4.

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

For more information about the 3.13.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

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>From 1ee15323059f2e44b0f76e23e699fcd5a1530e43 Mon Sep 17 00:00:00 2001
From: Alexei Starovoitov <ast at plumgrid.com>
Date: Tue, 13 May 2014 15:05:55 -0700
Subject: net: filter: x86: fix JIT address randomization

[ Upstream commit 773cd38f40b8834be991dbfed36683acc1dd41ee ]

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 >= PAGE_SIZE - sizeof(*header)
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 making sure that &header->image[prandom_u32() % hole] and &header
are in the same page

Fixes: 314beb9bcabfd ("x86: bpf_jit_comp: secure bpf jit against spraying attacks")
Signed-off-by: Alexei Starovoitov <ast at plumgrid.com>
Acked-by: Eric Dumazet <edumazet at google.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 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 4ed75dd..af2d431 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 - sizeof(*header));

 	/* insert a random number of int3 instructions before BPF code */
 	*image_ptr = &header->image[prandom_u32() % hole];
--
1.9.1