Public bug reported:
SRU Justification: This patch has been accepted into the upstream 2.6.27.4
stable kernel.
It should be pulled into the Ubuntu kernel as well.
TEST CASE: TBD
commit 1ea93b4aadc022277ddf92cf2aaa9ab00ca9a9b1
Author: Mikulas Patocka <[EMAIL PROTECTED]>
Date: Thu Oct 23 17:35:22 2008 +0000
dm snapshot: fix primary_pe race
commit 7c5f78b9d7f21937e46c26db82976df4b459c95c upstream
Fix a race condition with primary_pe ref_count handling.
put_pending_exception runs under dm_snapshot->lock, it does
atomic_dec_and_test
on primary_pe->ref_count, and later does atomic_read primary_pe->ref_count.
__origin_write does atomic_dec_and_test on primary_pe->ref_count without
holding
dm_snapshot->lock.
This opens the following race condition:
Assume two CPUs, CPU1 is executing put_pending_exception (and holding
dm_snapshot->lock). CPU2 is executing __origin_write in parallel.
primary_pe->ref_count == 2.
CPU1:
if (primary_pe && atomic_dec_and_test(&primary_pe->ref_count))
origin_bios = bio_list_get(&primary_pe->origin_bios);
.. decrements primary_pe->ref_count to 1. Doesn't load origin_bios
CPU2:
if (first && atomic_dec_and_test(&primary_pe->ref_count)) {
flush_bios(bio_list_get(&primary_pe->origin_bios));
free_pending_exception(primary_pe);
/* If we got here, pe_queue is necessarily empty. */
return r;
}
.. decrements primary_pe->ref_count to 0, submits pending bios, frees
primary_pe.
CPU1:
if (!primary_pe || primary_pe != pe)
free_pending_exception(pe);
.. this has no effect.
if (primary_pe && !atomic_read(&primary_pe->ref_count))
free_pending_exception(primary_pe);
.. sees ref_count == 0 (written by CPU 2), does double free !!
This bug can happen only if someone is simultaneously writing to both the
origin and the snapshot.
If someone is writing only to the origin, __origin_write will submit kcopyd
request after it decrements primary_pe->ref_count (so it can't happen that
the
finished copy races with primary_pe->ref_count decrementation).
If someone is writing only to the snapshot, __origin_write isn't invoked at
all
and the race can't happen.
The race happens when someone writes to the snapshot --- this creates
pending_exception with primary_pe == NULL and starts copying. Then, someone
writes to the same chunk in the snapshot, and __origin_write races with
termination of already submitted request in pending_complete (that calls
put_pending_exception).
This race may be reason for bugs:
http://bugzilla.kernel.org/show_bug.cgi?id=11636
https://bugzilla.redhat.com/show_bug.cgi?id=465825
The patch fixes the code to make sure that:
1. If atomic_dec_and_test(&primary_pe->ref_count) returns false, the process
must no longer dereference primary_pe (because someone else may free it
under
us).
2. If atomic_dec_and_test(&primary_pe->ref_count) returns true, the process
is responsible for freeing primary_pe.
Signed-off-by: Mikulas Patocka <[EMAIL PROTECTED]>
Signed-off-by: Alasdair G Kergon <[EMAIL PROTECTED]>
Signed-off-by: Greg Kroah-Hartman <[EMAIL PROTECTED]>
** Affects: linux (Ubuntu)
Importance: Undecided
Status: New
--
dm snapshot: fix primary_pe race
https://bugs.launchpad.net/bugs/294009
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