Re: [PATCH 3/3] ext4: mballoc: Fix spectre gadget in ext4_mb_simple_scan_group

[Josh Poimboeuf <jpoimboe@redhat.com>]

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.

-- 
Josh