Re: [PATCH 4.19] ext4: fix race condition between ext4_ioctl_setflags and ext4_fiemap

[Baokun Li]

在 2022/7/15 22:10, Greg KH 写道:
> On Fri, Jul 15, 2022 at 10:39:28AM +0800, Baokun Li wrote:
> > This patch and problem analysis is based on v4.19 LTS.
> > The d3b6f23f7167("ext4: move ext4_fiemap to use iomap framework") patch
> > is incorporated in v5.7-rc1. This patch avoids this problem by switching
> > to iomap in ext4_fiemap.
> >
> > Hulk Robot reported a BUG on stable 4.19.252:
> > ==================================================================
> > kernel BUG at fs/ext4/extents_status.c:762!
> > invalid opcode: 0000 [#1] SMP KASAN PTI
> > CPU: 7 PID: 2845 Comm: syz-executor Not tainted 4.19.252 #46
> > RIP: 0010:ext4_es_cache_extent+0x30e/0x370
> > [...]
> > Call Trace:
> >   ext4_cache_extents+0x238/0x2f0
> >   ext4_find_extent+0x785/0xa40
> >   ext4_fiemap+0x36d/0xe90
> >   do_vfs_ioctl+0x6af/0x1200
> > [...]
> > ==================================================================
> >
> > Above issue may happen as follows:
> > -------------------------------------
> >             cpu1		    cpu2
> > _____________________|_____________________
> > do_vfs_ioctl
> >   ext4_ioctl
> >    ext4_ioctl_setflags
> >     ext4_ind_migrate
> >                          do_vfs_ioctl
> >                           ioctl_fiemap
> >                            ext4_fiemap
> >                             ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS)
> >                             ext4_fill_fiemap_extents
> >      down_write(&EXT4_I(inode)->i_data_sem);
> >      ext4_ext_check_inode
> >      ext4_clear_inode_flag(inode, EXT4_INODE_EXTENTS)
> >      memset(ei->i_data, 0, sizeof(ei->i_data))
> >      up_write(&EXT4_I(inode)->i_data_sem);
> >                              down_read(&EXT4_I(inode)->i_data_sem);
> >                              ext4_find_extent
> >                               ext4_cache_extents
> >                                ext4_es_cache_extent
> >                                 BUG_ON(end < lblk)
> >
> > We can easily reproduce this problem with the syzkaller testcase:
> > ```
> > 02:37:07 executing program 3:
> > r0 = openat(0xffffffffffffff9c, &(0x7f0000000040)='./file0\x00', 0x26e1, 0x0)
> > ioctl$FS_IOC_FSSETXATTR(r0, 0x40086602, &(0x7f0000000080)={0x17e})
> > mkdirat(0xffffffffffffff9c, &(0x7f00000000c0)='./file1\x00', 0x1ff)
> > r1 = openat(0xffffffffffffff9c, &(0x7f0000000100)='./file1\x00', 0x0, 0x0)
> > ioctl$FS_IOC_FIEMAP(r1, 0xc020660b, &(0x7f0000000180)={0x0, 0x1, 0x0, 0xef3, 0x6, []}) (async, rerun: 32)
> > ioctl$FS_IOC_FSSETXATTR(r1, 0x40086602, &(0x7f0000000140)={0x17e}) (rerun: 32)
> > ```
> >
> > To solve this issue, we use __generic_block_fiemap() instead of
> > generic_block_fiemap() and add inode_lock_shared to avoid race condition.
> >
> > Reported-by: Hulk Robot <hulkci@xxxxxxxxxx>
> > Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
> > ---
> >   fs/ext4/extents.c | 15 +++++++++++----
> >   1 file changed, 11 insertions(+), 4 deletions(-)
> What is the git commit id of this change in Linus's tree?
>
> If it is not in Linus's tree, why not?
>
> confused,
>
> greg k-h
> .

This patch does not exist in the Linus' tree.

This problem persists until the patch d3b6f23f7167("ext4: move 
ext4_fiemap to use iomap framework") is incorporated in v5.7-rc1.

However, this problem can be found by asserting  after patch 
4068664e3cd2 ("ext4: fix extent_status fragmentation for plain files") 
is incorporated into v5.6-rc1.

If someone don't want to convert ext4_fiemap to iomap, this patch may help.

I also CC linux-ext4 in hopes of getting some advice.

Feel free to improve it if something wrong.

Thanks a lot!

--
With Best Regards,
Baokun Li