On Fri, Jul 27, 2018 at 04:23:57PM +0000, Jeremy Cline wrote:

'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 jpoimboe@redhat.com Cc: stable@vger.kernel.org Signed-off-by: Jeremy Cline jcline@redhat.com

---

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.