linux-ext4 - [PATCHv2] iomap: Fix negative assignment to unsigned sis->pages in iomap_swapfile

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Message-Id: <93fa3d674cda41261e529e1e9b75c2efb2e325be.1615445004.git.riteshh@linux.ibm.com>
Date:   Thu, 11 Mar 2021 12:24:26 +0530
From:   Ritesh Harjani <riteshh@...ux.ibm.com>
To:     linux-xfs@...r.kernel.org, linux-ext4@...r.kernel.org
Cc:     linux-fsdevel@...r.kernel.org, anju@...ux.vnet.ibm.com,
        "Darrick J . Wong" <djwong@...nel.org>,
        Ritesh Harjani <riteshh@...ux.ibm.com>
Subject: [PATCHv2] iomap: Fix negative assignment to unsigned sis->pages in iomap_swapfile_activate

In case if isi.nr_pages is 0, we are making sis->pages (which is
unsigned int) a huge value in iomap_swapfile_activate() by assigning -1.
This could cause a kernel crash in kernel v4.18 (with below signature).
This also crashes the upstream kernel with below signature, if the big fake
swap gets used.

Fix this issue by returning -EINVAL in case of nr_pages is 0, since it
is anyway a invalid swapfile. Looks like this issue will be hit when
we have blocksize < pagesize type of configuration.

I was able to hit the issue in case of a tiny swap file with below
test script.
https://raw.githubusercontent.com/riteshharjani/LinuxStudy/master/scripts/swap-issue.sh

<kernel crash on upstream kernel>
===============================
[  120.682527] XFS (loop2): Mounting V5 Filesystem
[  120.688448] XFS (loop2): Ending clean mount
[  120.690555] xfs filesystem being mounted at /mnt1/test supports timestamps until 2038 (0x7fffffff)
[  120.885793] Adding 4294967232k swap on /mnt1/test/swapfile.  Priority:-2 extents:1 across:274877906880k
<...>
[  186.068492] __swap_info_get: Bad swap offset entry 00000001
[  186.071704] __swap_info_get: Bad swap offset entry 00000043
<...>
[  453.756321] Faulting instruction address: 0xc0000000005b6c50
cpu 0x6: Vector: 300 (Data Access) at [c00000002a8b6f80]
    pc: c0000000005b6c50: __mark_inode_dirty+0x40/0x870
    lr: c0000000006435b0: iomap_set_page_dirty+0x170/0x1b0
    pid   = 4635, comm = stress
Linux version 5.12.0-rc1-00021-g23cdd4c7150 (gcc (Ubuntu 8.4.0-1ubuntu1~18.04)
iomap_set_page_dirty+0x170/0x1b0
swap_set_page_dirty+0xec/0x140
set_page_dirty+0x1b4/0x2d0
add_to_swap+0x178/0x1d0
shrink_page_list+0xe78/0x2120
shrink_inactive_list+0x2b0/0x640
shrink_lruvec+0x710/0x7b0
shrink_node+0x584/0x8e0
do_try_to_free_pages+0x2f8/0x5d0
<...>

kernel crash analysis on v4.18
==============================
On v4.18 kernel, it causes a kernel panic, since sis->pages becomes
a huge value and isi.nr_extents is 0. When 0 is returned it is
considered as a swapfile over NFS and SWP_FILE is set (sis->flags |= SWP_FILE).
Then when swapoff was getting called it was calling a_ops->swap_deactivate()
if (sis->flags & SWP_FILE) is true. Since a_ops->swap_deactivate() is
NULL in case of XFS, it causes below panic.

Panic signature on v4.18 kernel:
=======================================
[ 8291.723351] XFS (loop2): Unmounting Filesystem
[ 8292.123104] XFS (loop2): Mounting V5 Filesystem
[ 8292.132451] XFS (loop2): Ending clean mount
[ 8292.263362] Adding 4294967232k swap on /mnt1/test/swapfile.  Priority:-2 extents:1 across:274877906880k
[ 8292.277834] Unable to handle kernel paging request for instruction fetch
[ 8292.278677] Faulting instruction address: 0x00000000
cpu 0x19: Vector: 400 (Instruction Access) at [c0000009dd5b7ad0]
    pc: 0000000000000000
    lr: c0000000003eb9dc: destroy_swap_extents+0xfc/0x120
    pid   = 5604, comm = swapoff
Linux version 4.18.0 (riteshh@...xxxx) (gcc version 8.4.0 (Ubuntu 8.4.0-1ubuntu1~18.04)) #57 SMP Wed Mar 3 01:33:04 CST 2021
[link register   ] c0000000003eb9dc destroy_swap_extents+0xfc/0x120
[c0000009dd5b7d50] c0000000025a7058 proc_poll_event+0x0/0x4 (unreliable)
[c0000009dd5b7da0] c0000000003f0498 sys_swapoff+0x3f8/0x910
[c0000009dd5b7e30] c00000000000bbe4 system_call+0x5c/0x70
--- Exception: c01 (System Call) at 00007ffff7d208d8

Signed-off-by: Ritesh Harjani <riteshh@...ux.ibm.com>
---
 fs/iomap/swapfile.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/iomap/swapfile.c b/fs/iomap/swapfile.c
index a648dbf6991e..23e44bf97c65 100644
--- a/fs/iomap/swapfile.c
+++ b/fs/iomap/swapfile.c
@@ -170,6 +170,16 @@ int iomap_swapfile_activate(struct swap_info_struct *sis,
 			return ret;
 	}

+	/*
+	 * If this swapfile doesn't contain even a single page-aligned
+	 * contiguous range of blocks, reject this useless swapfile to
+	 * prevent confusion later on.
+	 */
+	if (isi.nr_pages == 0) {
+		pr_warn("swapon: Empty swap-file\n");
+		return -EINVAL;
+	}
+
 	*pagespan = 1 + isi.highest_ppage - isi.lowest_ppage;
 	sis->max = isi.nr_pages;
 	sis->pages = isi.nr_pages - 1;
--
2.26.2