CVE-2024-26759

In the Linux kernel, the following vulnerability has been resolved:
mm/swap: fix race when skipping swapcache When skipping swapcache for
SWP_SYNCHRONOUS_IO, if two or more threads swapin the same entry at the
same time, they get different pages (A, B). Before one thread (T0) finishes
the swapin and installs page (A) to the PTE, another thread (T1) could
finish swapin of page (B), swap_free the entry, then swap out the possibly
modified page reusing the same entry. It breaks the pte_same check in (T0)
because PTE value is unchanged, causing ABA problem. Thread (T0) will
install a stalled page (A) into the PTE and cause data corruption. One
possible callstack is like this: CPU0 CPU1 ---- ---- do_swap_page()
do_swap_page() with same entry <direct swapin path> <direct swapin path>
<alloc page A> <alloc page B> swap_read_folio() <- read to page A
swap_read_folio() <- read to page B <slow on later locks or interrupt>
<finished swapin first> … set_pte_at() swap_free() <- entry is free
<write to page B, now page A stalled> <swap out page B to same swap entry>
pte_same() <- Check pass, PTE seems unchanged, but page A is stalled!
swap_free() <- page B content lost! set_pte_at() <- staled page A
installed! And besides, for ZRAM, swap_free() allows the swap device to
discard the entry content, so even if page (B) is not modified, if
swap_read_folio() on CPU0 happens later than swap_free() on CPU1, it may
also cause data loss. To fix this, reuse swapcache_prepare which will pin
the swap entry using the cache flag, and allow only one thread to swap it
in, also prevent any parallel code from putting the entry in the cache.
Release the pin after PT unlocked. Racers just loop and wait since it’s a
rare and very short event. A schedule_timeout_uninterruptible(1) call is
added to avoid repeated page faults wasting too much CPU, causing livelock
or adding too much noise to perf statistics. A similar livelock issue was
described in commit 029c4628b2eb (“mm: swap: get rid of livelock in swapin
readahead”) Reproducer: This race issue can be triggered easily using a
well constructed reproducer and patched brd (with a delay in read path)
[1]: With latest 6.8 mainline, race caused data loss can be observed
easily: $ gcc -g -lpthread test-thread-swap-race.c && ./a.out Polulating
32MB of memory region… Keep swapping out… Starting round 0… Spawning
65536 workers… 32746 workers spawned, wait for done… Round 0: Error on
0x5aa00, expected 32746, got 32743, 3 data loss! Round 0: Error on
0x395200, expected 32746, got 32743, 3 data loss! Round 0: Error on
0x3fd000, expected 32746, got 32737, 9 data loss! Round 0 Failed, 15 data
loss! This reproducer spawns multiple threads sharing the same memory
region using a small swap device. Every two threads updates mapped pages
one by one in opposite direction trying to create a race, with one
dedicated thread keep swapping out the data out using madvise. The
reproducer created a reproduce rate of about once every 5 minutes, so the
race should be totally possible in production. After this patch, I ran the
reproducer for over a few hundred rounds and no data loss observed.
Performance overhead is minimal, microbenchmark swapin 10G from 32G zram:
Before: 10934698 us After: 11157121 us Cached: 13155355 us (Dropping
SWP_SYNCHRONOUS_IO flag) [[email protected]: v4] Link:
https://lkml.kernel.org/r/[email protected]