fs/ext4: fix integer overflow in s_log_groups_per_flex

syzbot found UBSAN: shift-out-of-bounds in ext4_mb_init [1], when
1 << sbi->s_es->s_log_groups_per_flex is bigger than UINT_MAX,
where sbi->s_mb_prefetch is unsigned integer type.

32 is the maximum allowed power of s_log_groups_per_flex. Following if
check will also trigger UBSAN shift-out-of-bound:

if (1 << sbi->s_es->s_log_groups_per_flex >= UINT_MAX) {

So I'm checking it against the raw number, perhaps there is another way
to calculate UINT_MAX max power. Also use min_t as to make sure it's
uint type.

[1] UBSAN: shift-out-of-bounds in fs/ext4/mballoc.c:2713:24
shift exponent 60 is too large for 32-bit type 'int'
Call Trace:
 __dump_stack lib/dump_stack.c:79 [inline]
 dump_stack+0x137/0x1be lib/dump_stack.c:120
 ubsan_epilogue lib/ubsan.c:148 [inline]
 __ubsan_handle_shift_out_of_bounds+0x432/0x4d0 lib/ubsan.c:395
 ext4_mb_init_backend fs/ext4/mballoc.c:2713 [inline]
 ext4_mb_init+0x19bc/0x19f0 fs/ext4/mballoc.c:2898
 ext4_fill_super+0xc2ec/0xfbe0 fs/ext4/super.c:4983

Reported-by: syzbot+a8b4b0c60155e87e9484@syzkaller.appspotmail.com
Signed-off-by: Sabyrzhan Tasbolatov <snovitoll@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20210224095800.3350002-1-snovitoll@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 99bf091..a02fadf 100644 (file)
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -2709,8 +2709,15 @@ static int ext4_mb_init_backend(struct super_block *sb)
        }
 
        if (ext4_has_feature_flex_bg(sb)) {
-               /* a single flex group is supposed to be read by a single IO */
-               sbi->s_mb_prefetch = min(1 << sbi->s_es->s_log_groups_per_flex,
+               /* a single flex group is supposed to be read by a single IO.
+                * 2 ^ s_log_groups_per_flex != UINT_MAX as s_mb_prefetch is
+                * unsigned integer, so the maximum shift is 32.
+                */
+               if (sbi->s_es->s_log_groups_per_flex >= 32) {
+                       ext4_msg(sb, KERN_ERR, "too many log groups per flexible block group");
+                       goto err_freesgi;
+               }
+               sbi->s_mb_prefetch = min_t(uint, 1 << sbi->s_es->s_log_groups_per_flex,
                        BLK_MAX_SEGMENT_SIZE >> (sb->s_blocksize_bits - 9));
                sbi->s_mb_prefetch *= 8; /* 8 prefetch IOs in flight at most */
        } else {