Re: [PATCH 3/3] ext4: mballoc: Fix spectre gadget in ext4_mb_simple_scan_group
From: Josh Poimboeuf <hidden>
Date: 2018-07-27 18:10:32
Also in:
lkml, stable
On Fri, Jul 27, 2018 at 04:23:57PM +0000, Jeremy Cline wrote:
quoted hunk ↗ jump to hunk
'ac->ac_2order' is a user-controlled value used to index into 'grp->bb_counters' and based on the value at that index, 'ac->ac_found' is written to. Clamp the value right after the bounds check to avoid a speculative out-of-bounds read of 'grp->bb_counters'. This also protects the access of the s_mb_offsets and s_mb_maxs arrays inside mb_find_buddy(). These gadgets were discovered 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 <redacted> Cc: stable@vger.kernel.org Signed-off-by: Jeremy Cline <redacted> --- fs/ext4/mballoc.c | 2 ++ 1 file changed, 2 insertions(+)diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c index f7ab34088162..c0866007a949 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>@@ -1893,6 +1894,7 @@ void ext4_mb_simple_scan_group(struct ext4_allocation_context *ac, BUG_ON(ac->ac_2order <= 0); for (i = ac->ac_2order; i <= sb->s_blocksize_bits + 1; i++) { + i = array_index_nospec(i, sb->s_blocksize_bits + 2); if (grp->bb_counters[i] == 0) continue;
Similar to my patch 1 comment, it's better to go up the call chain.
ac_2order's user taint seems to come from ext4_mb_regular_allocator(),
where it's derived from ac->ac_g_ex.fe_len, which has a user taint
according to smatch.
i = fls(ac->ac_g_ex.fe_len);
ac->ac_2order = 0;
/*
* We search using buddy data only if the order of the request
* is greater than equal to the sbi_s_mb_order2_reqs
* You can tune it via /sys/fs/ext4/<partition>/mb_order2_req
* We also support searching for power-of-two requests only for
* requests upto maximum buddy size we have constructed.
*/
if (i >= sbi->s_mb_order2_reqs && i <= sb->s_blocksize_bits + 2) {
/*
* 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;
}
So here maybe you could change the assignment to:
ac->ac_2order = array_index_nospec(i - 1,
sb->s_blocksize_bits + 2);
That makes it easier for a reader of the code to understand what
speculation we're protecting against. And it also protects other
consumers of this value down the call chain.
--
Josh