linux-ext4 - [PATCH v2] ext4: mballoc: Fix spectre gadget in ext4_mb_regular

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Date:   Mon, 30 Jul 2018 18:07:47 +0000
From:   Jeremy Cline <jcline@...hat.com>
To:     Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>
Cc:     linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jeremy Cline <jcline@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>, stable@...r.kernel.org
Subject: [PATCH v2] ext4: mballoc: Fix spectre gadget in ext4_mb_regular_allocator

'ac->ac_g_ex.fe_len' is a user-controlled value which is used in the
derivation of 'ac->ac_2order'. 'ac->ac_2order', in turn, is used to
index arrays which makes it a potential spectre gadget. Fix this by
sanitizing the value assigned to 'ac->ac2_order'.  This covers the
following accesses found with the help of smatch:

* fs/ext4/mballoc.c:1896 ext4_mb_simple_scan_group() warn: potential
  spectre issue 'grp->bb_counters' [w] (local cap)

* fs/ext4/mballoc.c:445 mb_find_buddy() warn: potential spectre issue
  'EXT4_SB(e4b->bd_sb)->s_mb_offsets' [r] (local cap)

* fs/ext4/mballoc.c:446 mb_find_buddy() warn: potential spectre issue
  'EXT4_SB(e4b->bd_sb)->s_mb_maxs' [r] (local cap)

Cc: Josh Poimboeuf <jpoimboe@...hat.com>
Cc: stable@...r.kernel.org
Suggested-by: Josh Poimboeuf <jpoimboe@...hat.com>
Signed-off-by: Jeremy Cline <jcline@...hat.com>
---

I broke this out of the "ext4: fix spectre v1 gadgets" patch set since
the other patches in that series could, as Josh noted, be replaced with
one fix in do_quotactl. I'll send that fix to the disk quota folks
separately.

Changes from v1:
  - Sanitize ac_2order on assignment, rather than down the call chain in
    ext4_mb_simple_scan_group.

 fs/ext4/mballoc.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index f7ab34088162..8b24d3d42cb3 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -14,6 +14,7 @@
 #include <linux/log2.h>
 #include <linux/module.h>
 #include <linux/slab.h>
+#include <linux/nospec.h>
 #include <linux/backing-dev.h>
 #include <trace/events/ext4.h>
 
@@ -2140,7 +2141,8 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
 		 * This should tell if fe_len is exactly power of 2
 		 */
 		if ((ac->ac_g_ex.fe_len & (~(1 << (i - 1)))) == 0)
-			ac->ac_2order = i - 1;
+			ac->ac_2order = array_index_nospec(i - 1,
+							   sb->s_blocksize_bits + 2);
 	}
 
 	/* if stream allocation is enabled, use global goal */
-- 
2.17.1