On 2025-08-25 12:02, Ritesh Harjani (IBM) wrote: > libaokun@xxxxxxxxxxxxxxx writes: > >> From: Baokun Li <libaokun1@xxxxxxxxxx> >> >> In ext4_mb_init(), ext4_mb_avg_fragment_size_destroy() may be called >> when sbi->s_mb_avg_fragment_size remains uninitialized (e.g., if groupinfo >> slab cache allocation fails). Since ext4_mb_avg_fragment_size_destroy() >> lacks null pointer checking, this leads to a null pointer dereference. >> >> ================================================================== >> EXT4-fs: no memory for groupinfo slab cache >> BUG: kernel NULL pointer dereference, address: 0000000000000000 >> PGD 0 P4D 0 >> Oops: Oops: 0002 [#1] SMP PTI >> CPU:2 UID: 0 PID: 87 Comm:mount Not tainted 6.17.0-rc2 #1134 PREEMPT(none) >> RIP: 0010:_raw_spin_lock_irqsave+0x1b/0x40 >> Call Trace: >> <TASK> >> xa_destroy+0x61/0x130 >> ext4_mb_init+0x483/0x540 >> __ext4_fill_super+0x116d/0x17b0 >> ext4_fill_super+0xd3/0x280 >> get_tree_bdev_flags+0x132/0x1d0 >> vfs_get_tree+0x29/0xd0 >> do_new_mount+0x197/0x300 >> __x64_sys_mount+0x116/0x150 >> do_syscall_64+0x50/0x1c0 >> entry_SYSCALL_64_after_hwframe+0x76/0x7e >> ================================================================== >> >> Therefore, add necessary null check to ext4_mb_avg_fragment_size_destroy() >> to prevent this issue. The same fix is also applied to >> ext4_mb_largest_free_orders_destroy(). >> >> Reported-by: syzbot+1713b1aa266195b916c2@xxxxxxxxxxxxxxxxxxxxxxxxx >> Closes: https://syzkaller.appspot.com/bug?extid=1713b1aa266195b916c2 >> Fixes: f7eaacbb4e54 ("ext4: convert free groups order lists to xarrays") >> Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx> >> --- >> fs/ext4/mballoc.c | 10 ++++++++++ >> 1 file changed, 10 insertions(+) > I was just looking at the syzbot report. You submitted the fix pretty > fast ;) Since this code was introduced by me, when I saw the syzbot report, I immediately realized that my previous changes had unintentionally removed the null pointer check... > Since these two functions can get called in error handling routine, we > should check for null before iterating over the uninitalized array. Yes, when making the changes, I missed the implicit null pointer check within kfree. Sorry for this oversight. > > The fix looks good to me. Please feel free to add: > Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@xxxxxxxxx> > > -ritesh > Thanks for the review! Cheers, Baokun >> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c >> index 5898d92ba19f..6070d3c86678 100644 >> --- a/fs/ext4/mballoc.c >> +++ b/fs/ext4/mballoc.c >> @@ -3655,16 +3655,26 @@ static void ext4_discard_work(struct work_struct *work) >> >> static inline void ext4_mb_avg_fragment_size_destroy(struct ext4_sb_info *sbi) >> { >> + if (!sbi->s_mb_avg_fragment_size) >> + return; >> + >> for (int i = 0; i < MB_NUM_ORDERS(sbi->s_sb); i++) >> xa_destroy(&sbi->s_mb_avg_fragment_size[i]); >> + >> kfree(sbi->s_mb_avg_fragment_size); >> + sbi->s_mb_avg_fragment_size = NULL; >> } >> >> static inline void ext4_mb_largest_free_orders_destroy(struct ext4_sb_info *sbi) >> { >> + if (!sbi->s_mb_largest_free_orders) >> + return; >> + >> for (int i = 0; i < MB_NUM_ORDERS(sbi->s_sb); i++) >> xa_destroy(&sbi->s_mb_largest_free_orders[i]); >> + >> kfree(sbi->s_mb_largest_free_orders); >> + sbi->s_mb_largest_free_orders = NULL; >> } >> >> int ext4_mb_init(struct super_block *sb) >> -- >> 2.46.1
