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Subjectzram corruption due to uninitialized do_swap_page fault
Hello,

We're looking into using zram, but unfortunately we ran into some
corruption issues. We've seen rocksdb complaining about "Corruption:
bad entry in block", and we've also seen some coredumps that point at
memory being zeroed out. One of our Rust processes coredumps contains
a non-null pointer pointing at zero, among other things:

* core::ptr::non_null::NonNull<u8> {pointer: 0x0}

In fact, a whole bunch of memory around this pointer was all zeros.

Disabling zram resolves all issues, and we can't reproduce any of
these issues with other swap setups. I've tried adding crc32
checksumming for pages that are compressed, but it didn't catch the
issue either, even though userspace facing symptoms were present. My
crc32 code doesn't touch ZRAM_SAME pages, though.

Unfortunately, this isn't trivial to replicate, and I believe that it
depends on zram used for swap specifically, not for zram as a block
device. Specifically, swap_slot_free_notify looks suspicious.

Here's a patch that I have to catch the issue in the act:

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 438ce34ee760..fea46a70a3c9 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -1265,6 +1265,9 @@ static int __zram_bvec_read(struct zram *zram,
struct page *page, u32 index,
unsigned long value;
void *mem;

+ if (WARN_ON(!handle && !zram_test_flag(zram, index, ZRAM_SAME)))
+ pr_warn("Page %u read from zram without previous write\n", index);
+
value = handle ? zram_get_element(zram, index) : 0;
mem = kmap_atomic(page);
zram_fill_page(mem, PAGE_SIZE, value);
In essence, it warns whenever a page is read from zram that was not
previously written to. To make this work, one needs to zero out zram
prior to running mkswap on it.

I have prepared a GitHub repo with my observations and a reproduction:

* https://github.com/bobrik/zram-corruptor

I'm able to trigger the following in an aarch64 VM with two threads
reading the same memory out of swap:

[ 512.651752][ T7285] ------------[ cut here ]------------
[ 512.652279][ T7285] WARNING: CPU: 0 PID: 7285 at
drivers/block/zram/zram_drv.c:1285 __zram_bvec_read+0x28c/0x2e8 [zram]
[ 512.653923][ T7285] Modules linked in: zram zsmalloc kheaders nfsv3
nfs lockd grace sunrpc xt_conntrack nft_chain_nat xt_MASQUERADE nf_nat
nf_conntrack_netlink nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4
nft_counter xt_addrtype nft_compat nf_tables nfnetlink bridge stp llc
overlay xfs libcrc32c zstd zstd_compress af_packet aes_ce_blk
aes_ce_cipher ghash_ce gf128mul virtio_net sha3_ce net_failover
sha3_generic failover sha512_ce sha512_arm64 sha2_ce sha256_arm64
virtio_mmio virtio_ring qemu_fw_cfg rtc_pl031 virtio fuse ip_tables
x_tables ext4 mbcache crc16 jbd2 nvme nvme_core pci_host_generic
pci_host_common unix [last unloaded: zsmalloc]
[ 512.659238][ T7285] CPU: 0 PID: 7285 Comm: zram-corruptor Tainted: G
W 5.16.0-ivan #1 0877d306c6dc0716835d43cafe4399473d09e406
[ 512.660413][ T7285] Hardware name: linux,dummy-virt (DT)
[ 512.661077][ T7285] pstate: 80400005 (Nzcv daif +PAN -UAO -TCO -DIT
-SSBS BTYPE=--)
[ 512.661788][ T7285] pc : __zram_bvec_read+0x28c/0x2e8 [zram]
[ 512.662099][ T7285] lr : zram_bvec_rw+0x70/0x204 [zram]
[ 512.662422][ T7285] sp : ffffffc01018bac0
[ 512.662720][ T7285] x29: ffffffc01018bae0 x28: ffffff9e4e725280 x27:
ffffff9e4e725280
[ 512.663122][ T7285] x26: ffffff9e4e725280 x25: 00000000000001f6 x24:
0000000100033e6c
[ 512.663601][ T7285] x23: 00000000000001f6 x22: 0000000000000000 x21:
fffffffe7a36d840
[ 512.664252][ T7285] x20: 00000000000001f6 x19: ffffff9e69423c00 x18:
ffffffc010711068
[ 512.664812][ T7285] x17: 0000000000000008 x16: ffffffd34aed51bc x15:
0000000000000000
[ 512.665507][ T7285] x14: 0000000000000a88 x13: 0000000000000000 x12:
0000000000000000
[ 512.666183][ T7285] x11: 0000000100033e6c x10: ffffffc01091d000 x9 :
0000000001000000
[ 512.666627][ T7285] x8 : 0000000000002f10 x7 : 80b75f8fb90b52c4 x6 :
051609fe50833de3
[ 512.667276][ T7285] x5 : 0000000000000000 x4 : 0000000000000000 x3 :
0000000000000000
[ 512.667875][ T7285] x2 : 00000000000001f6 x1 : 00000000000001f6 x0 :
ffffffd305b746af
[ 512.668483][ T7285] Call trace:
[ 512.668682][ T7285] __zram_bvec_read+0x28c/0x2e8 [zram
745969ed35ea0fb382bfd518d6f70e13966e9b52]
[ 512.669405][ T7285] zram_bvec_rw+0x70/0x204 [zram
745969ed35ea0fb382bfd518d6f70e13966e9b52]
[ 512.670066][ T7285] zram_rw_page+0xb4/0x16c [zram
745969ed35ea0fb382bfd518d6f70e13966e9b52]
[ 512.670584][ T7285] bdev_read_page+0x74/0xac
[ 512.670843][ T7285] swap_readpage+0x5c/0x2e4
[ 512.671243][ T7285] do_swap_page+0x2f4/0x988
[ 512.671560][ T7285] handle_pte_fault+0xcc/0x1fc
[ 512.671935][ T7285] handle_mm_fault+0x284/0x4a8
[ 512.672412][ T7285] do_page_fault+0x274/0x428
[ 512.672704][ T7285] do_translation_fault+0x5c/0xf8
[ 512.673083][ T7285] do_mem_abort+0x50/0xc8
[ 512.673293][ T7285] el0_da+0x3c/0x74
[ 512.673549][ T7285] el0t_64_sync_handler+0xc4/0xec
[ 512.673972][ T7285] el0t_64_sync+0x1a4/0x1a8
[ 512.674495][ T7285] ---[ end trace cf983b7507c20343 ]---
[ 512.675359][ T7285] zram: Page 502 read from zram without previous write
I can also trace accesses to zram to catch the unfortunate sequence:

zram_bvec_write index = 502 [cpu = 3, tid = 7286]
zram_free_page index = 502 [cpu = 3, tid = 7286]
zram_bvec_read index = 502 [cpu = 3, tid = 7286]
zram_free_page index = 502 [cpu = 3, tid = 7286] <-- problematic free
zram_bvec_read index = 502 [cpu = 0, tid = 7285] <-- problematic read

With stacks for zram_free_page:

zram_bvec_write index = 502 [cpu = 3, tid = 7286]
zram_free_page index = 502 [cpu = 3, tid = 7286]

zram_free_page+0
$x.97+32
zram_rw_page+180
bdev_write_page+124
__swap_writepage+116
swap_writepage+160
pageout+284
shrink_page_list+2892
shrink_inactive_list+688
shrink_lruvec+360
shrink_node_memcgs+148
shrink_node+860
shrink_zones+368
do_try_to_free_pages+232
try_to_free_mem_cgroup_pages+292
try_charge_memcg+608

zram_bvec_read index = 502 [cpu = 3, tid = 7286]
zram_free_page index = 502 [cpu = 3, tid = 7286] <-- problematic free

zram_free_page+0
swap_range_free+220
swap_entry_free+244
swapcache_free_entries+152
free_swap_slot+288
__swap_entry_free+216
swap_free+108
do_swap_page+1776
handle_pte_fault+204
handle_mm_fault+644
do_page_fault+628
do_translation_fault+92
do_mem_abort+80
el0_da+60
el0t_64_sync_handler+196
el0t_64_sync+420

zram_bvec_read index = 502 [cpu = 0, tid = 7285] <-- problematic read

The very last read is the same one that triggered the warning from my
patch in dmesg. You can see that the slot is freed before reading by
swapcache_free_entries. As far as I can see, only zram implements
swap_slot_free_notify. Swapping in an uninitialized zram page results
in all zeroes copied, which matches the symptoms.

The issue doesn't reproduce if I pin both threads to the same CPU. It
also doesn't reproduce with a single thread. All of this seems to
point at some sort of race condition.

I was able to reproduce this on x86_64 bare metal server as well.

I'm happy to try out mitigation approaches for this. If my
understanding here is incorrect, I'm also happy to try out patches
that could help me catch the issue in the wild.

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