Re: [PATCH 2/4] ext4: fix BUG in ext4_mb_new_inode_pa() due to overflow
From: Baokun Li
Date: Fri Jul 21 2023 - 04:02:09 EST
On 2023/7/21 3:07, Ritesh Harjani (IBM) wrote:
Baokun Li <libaokun1@xxxxxxxxxx> writes:
When we calculate the end position of ext4_free_extent, this position may
be exactly where ext4_lblk_t (i.e. uint) overflows. For example, if
ac_g_ex.fe_logical is 4294965248 and ac_orig_goal_len is 2048, then the
computed end is 0x100000000, which is 0. If ac->ac_o_ex.fe_logical is not
the first case of adjusting the best extent, that is, new_bex_end > 0, the
following BUG_ON will be triggered:
=========================================================
kernel BUG at fs/ext4/mballoc.c:5116!
invalid opcode: 0000 [#1] PREEMPT SMP PTI
CPU: 3 PID: 673 Comm: xfs_io Tainted: G E 6.5.0-rc1+ #279
RIP: 0010:ext4_mb_new_inode_pa+0xc5/0x430
Call Trace:
<TASK>
ext4_mb_use_best_found+0x203/0x2f0
ext4_mb_try_best_found+0x163/0x240
ext4_mb_regular_allocator+0x158/0x1550
ext4_mb_new_blocks+0x86a/0xe10
ext4_ext_map_blocks+0xb0c/0x13a0
ext4_map_blocks+0x2cd/0x8f0
ext4_iomap_begin+0x27b/0x400
iomap_iter+0x222/0x3d0
__iomap_dio_rw+0x243/0xcb0
iomap_dio_rw+0x16/0x80
=========================================================
A simple reproducer demonstrating the problem:
mkfs.ext4 -F /dev/sda -b 4096 100M
mount /dev/sda /tmp/test
fallocate -l1M /tmp/test/tmp
fallocate -l10M /tmp/test/file
fallocate -i -o 1M -l16777203M /tmp/test/file
fsstress -d /tmp/test -l 0 -n 100000 -p 8 &
sleep 10 && killall -9 fsstress
rm -f /tmp/test/tmp
xfs_io -c "open -ad /tmp/test/file" -c "pwrite -S 0xff 0 8192"
We declare new_bex_start and new_bex_end as correct types and use fex_end()
to avoid the problems caused by the ext4_lblk_t overflow above.
Fixes: 93cdf49f6eca ("ext4: Fix best extent lstart adjustment logic in ext4_mb_new_inode_pa()")
Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
---
fs/ext4/mballoc.c | 11 +++++------
1 file changed, 5 insertions(+), 6 deletions(-)
diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index eb7f5d35ef96..2090e5e7ba58 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -5076,8 +5076,8 @@ ext4_mb_new_inode_pa(struct ext4_allocation_context *ac)
pa = ac->ac_pa;
if (ac->ac_b_ex.fe_len < ac->ac_orig_goal_len) {
- int new_bex_start;
- int new_bex_end;
+ ext4_lblk_t new_bex_start;
+ loff_t new_bex_end;
/* we can't allocate as much as normalizer wants.
* so, found space must get proper lstart
@@ -5096,8 +5096,7 @@ ext4_mb_new_inode_pa(struct ext4_allocation_context *ac)
* still cover original start
* 3. Else, keep the best ex at start of original request.
*/
- new_bex_end = ac->ac_g_ex.fe_logical +
- EXT4_C2B(sbi, ac->ac_orig_goal_len);
+ new_bex_end = fex_end(sbi, &ac->ac_g_ex, &ac->ac_orig_goal_len);
new_bex_start = new_bex_end - EXT4_C2B(sbi, ac->ac_b_ex.fe_len);
if (ac->ac_o_ex.fe_logical >= new_bex_start)
goto adjust_bex;
new_bex_end = ac->ac_g_ex.fe_logical +
EXT4_C2B(sbi, ac->ac_orig_goal_len);
new_bex_start = new_bex_end - EXT4_C2B(sbi, ac->ac_b_ex.fe_len);
if (ac->ac_o_ex.fe_logical >= new_bex_start)
goto adjust_bex;
new_bex_start = ac->ac_g_ex.fe_logical;
new_bex_end =
new_bex_start + EXT4_C2B(sbi, ac->ac_b_ex.fe_len);
if (ac->ac_o_ex.fe_logical < new_bex_end)
goto adjust_bex;
new_bex_start = ac->ac_o_ex.fe_logical;
new_bex_end =
new_bex_start + EXT4_C2B(sbi, ac->ac_b_ex.fe_len);
I think it will be safer if we typecast all above new_bex_end
calculations. I understand that it might never happen here that
fe_logical + best found extent length will make it overflow.
Simply because we only enter here when ac_b_ex.fe_len <
ac_orig_goal_len. But I think if we change any logic tomorrow or
refactor any part of code, it will be much safer if we simply keep the
typecast in place so that we avoid any future tripping caused by
arithmatic overflow here.
Also I am not finding fex_end() or pa_end() helpers very intuitive way
of doing it. I feel typecasting in place is much simpler and reads better.
-ritesh
I remove the third parameter of fex_end() and adjust the code in
ext4_mb_new_inode_pa() as follows:
struct ext4_free_extent ex = {
.fe_logical = ac->ac_g_ex.fe_logical,
.fe_len = ac->ac_orig_goal_len,
};
loff_t orig_goal_end = fex_end(sbi, &ex);
ex.fe_len = ac->ac_b_ex.fe_len;
ex.fe_logical = orig_goal_end - EXT4_C2B(sbi, ex.fe_len);
if (ac->ac_o_ex.fe_logical >= ex.fe_logical)
goto adjust_bex;
ex.fe_logical = ac->ac_g_ex.fe_logical;
if (ac->ac_o_ex.fe_logical < fex_end(sbi, &ex))
goto adjust_bex;
ex.fe_logical = ac->ac_o_ex.fe_logical;
adjust_bex:
ac->ac_b_ex.fe_logical = ex.fe_logical;
BUG_ON(ac->ac_o_ex.fe_logical < ac->ac_b_ex.fe_logical);
BUG_ON(ac->ac_o_ex.fe_len > ac->ac_b_ex.fe_len);
BUG_ON(fex_end(sbi, &ex) > orig_goal_end);
In this way, all new_bex_end calculations do not have overflow problems,
and the code looks much more neat than before. 😊
@@ -5117,8 +5116,8 @@ ext4_mb_new_inode_pa(struct ext4_allocation_context *ac)
BUG_ON(ac->ac_o_ex.fe_logical < ac->ac_b_ex.fe_logical);
BUG_ON(ac->ac_o_ex.fe_len > ac->ac_b_ex.fe_len);
- BUG_ON(new_bex_end > (ac->ac_g_ex.fe_logical +
- EXT4_C2B(sbi, ac->ac_orig_goal_len)));
+ BUG_ON(new_bex_end >
+ fex_end(sbi, &ac->ac_g_ex, &ac->ac_orig_goal_len));
}
pa->pa_lstart = ac->ac_b_ex.fe_logical;
--
2.31.1
Thanks!
--
With Best Regards,
Baokun Li
.