[PATCH v2 00/10] mm/hugetlb: Make huge_pte_offset() thread-safe for pmd unshare

Peter Xu
2022-12-07T20:31:57Z

Based on latest mm-unstable (dbccafc6e992).

This can be seen as a follow-up series to Mike's recent hugetlb vma lock
series for pmd unsharing, but majorly covering safe use of huge_pte_offset.

This version should have all the issues resolved regarding v1.

Old versions:

rfcv1: https://lore.kernel.org/r/20221030212929.335473-1-peterx@redhat.com
rfcv2: https://lore.kernel.org/r/20221118011025.2178986-1-peterx@redhat.com
v1: https://lore.kernel.org/r/20221129193526.3588187-1-peterx@redhat.com

Changelog for v1->v2:
- Added a-bs and r-bs
- Dropped the old patch 9, replacing it with a new document update patch 10
- Patch 4
- Dropped obsolete comment in hugetlb_fault() regarding ptep [Mike]
- Patch 8
- Release vma lock and retake for three special cases, where two of them
can proactively yield (s390 skey, pagemap) and the 3rd one (HMM) can
call handle_mm_fault nested, so taking the vma lock nested.
- More comments
- Patch 9
- Fix compilation error when !HUGETLB_PAGE

Problem
=======

huge_pte_offset() is a major helper used by hugetlb code paths to walk a
hugetlb pgtable. It's used mostly everywhere since that's needed even
before taking the pgtable lock.

huge_pte_offset() is always called with mmap lock held with either read or
write. It was assumed to be safe but it's actually not. One race
condition can easily trigger by: (1) firstly trigger pmd share on a memory
range, (2) do huge_pte_offset() on the range, then at the meantime, (3)
another thread unshare the pmd range, and the pgtable page is prone to lost
if the other shared process wants to free it completely (by either munmap
or exit mm).

The recent work from Mike on vma lock can resolve most of this already.
It's achieved by forbidden pmd unsharing during the lock being taken, so no
further risk of the pgtable page being freed. It means if we can take the
vma lock around all huge_pte_offset() callers it'll be safe.

There're already a bunch of them that we did as per the latest mm-unstable,
but also quite a few others that we didn't for various reasons especially
on huge_pte_offset() usage.

One more thing to mention is that besides the vma lock, i_mmap_rwsem can
also be used to protect the pgtable page (along with its pgtable lock) from
being freed from under us. IOW, huge_pte_offset() callers need to either
hold the vma lock or i_mmap_rwsem to safely walk the pgtables.

A reproducer of such problem, based on hugetlb GUP (NOTE: since the race is
very hard to trigger, one needs to apply another kernel delay patch too,
see below):

======8<=======
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <unistd.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <linux/memfd.h>
#include <assert.h>
#include <pthread.h>

#define MSIZE (1UL << 30) /* 1GB */
#define PSIZE (2UL << 20) /* 2MB */

#define HOLD_SEC (1)

int pipefd[2];
void *buf;

void *do_map(int fd)
{
unsigned char *tmpbuf, *p;
int ret;

ret = posix_memalign((void **)&tmpbuf, MSIZE, MSIZE);
if (ret) {
perror("posix_memalign() failed");
return NULL;
}

tmpbuf = mmap(tmpbuf, MSIZE, PROT_READ | PROT_WRITE,
MAP_SHARED | MAP_FIXED, fd, 0);
if (tmpbuf == MAP_FAILED) {
perror("mmap() failed");
return NULL;
}
printf("mmap() -> %p\n", tmpbuf);

for (p = tmpbuf; p < tmpbuf + MSIZE; p += PSIZE) {
*p = 1;
}

return tmpbuf;
}

void do_unmap(void *buf)
{
munmap(buf, MSIZE);
}

void proc2(int fd)
{
unsigned char c;

buf = do_map(fd);
if (!buf)
return;

read(pipefd[0], &c, 1);
/*
* This frees the shared pgtable page, causing use-after-free in
* proc1_thread1 when soft walking hugetlb pgtable.
*/
do_unmap(buf);

printf("Proc2 quitting\n");
}

void *proc1_thread1(void *data)
{
/*
* Trigger follow-page on 1st 2m page. Kernel hack patch needed to
* withhold this procedure for easier reproduce.
*/
madvise(buf, PSIZE, MADV_POPULATE_WRITE);
printf("Proc1-thread1 quitting\n");
return NULL;
}

void *proc1_thread2(void *data)
{
unsigned char c;

/* Wait a while until proc1_thread1() start to wait */
sleep(0.5);
/* Trigger pmd unshare */
madvise(buf, PSIZE, MADV_DONTNEED);
/* Kick off proc2 to release the pgtable */
write(pipefd[1], &c, 1);

printf("Proc1-thread2 quitting\n");
return NULL;
}

void proc1(int fd)
{
pthread_t tid1, tid2;
int ret;

buf = do_map(fd);
if (!buf)
return;

ret = pthread_create(&tid1, NULL, proc1_thread1, NULL);
assert(ret == 0);
ret = pthread_create(&tid2, NULL, proc1_thread2, NULL);
assert(ret == 0);

/* Kick the child to share the PUD entry */
pthread_join(tid1, NULL);
pthread_join(tid2, NULL);

do_unmap(buf);
}

int main(void)
{
int fd, ret;

fd = memfd_create("test-huge", MFD_HUGETLB | MFD_HUGE_2MB);
if (fd < 0) {
perror("open failed");
return -1;
}

ret = ftruncate(fd, MSIZE);
if (ret) {
perror("ftruncate() failed");
return -1;
}

ret = pipe(pipefd);
if (ret) {
perror("pipe() failed");
return -1;
}

if (fork()) {
proc1(fd);
} else {
proc2(fd);
}

close(pipefd[0]);
close(pipefd[1]);
close(fd);

return 0;
}
======8<=======

The kernel patch needed to present such a race so it'll trigger 100%:

======8<=======
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 9d97c9a2a15d..f8d99dad5004 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -38,6 +38,7 @@
#include <asm/page.h>
#include <asm/pgalloc.h>
#include <asm/tlb.h>
+#include <asm/delay.h>

#include <linux/io.h>
#include <linux/hugetlb.h>
@@ -6290,6 +6291,7 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
bool unshare = false;
int absent;
struct page *page;
+ unsigned long c = 0;

/*
* If we have a pending SIGKILL, don't keep faulting pages and
@@ -6309,6 +6311,13 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
*/
pte = huge_pte_offset(mm, vaddr & huge_page_mask(h),
huge_page_size(h));
+
+ pr_info("%s: withhold 1 sec...\n", __func__);
+ for (c = 0; c < 100; c++) {
+ udelay(10000);
+ }
+ pr_info("%s: withhold 1 sec...done\n", __func__);
+
if (pte)
ptl = huge_pte_lock(h, mm, pte);
absent = !pte || huge_pte_none(huge_ptep_get(pte));
======8<=======

It'll trigger use-after-free of the pgtable spinlock:

======8<=======
[ 16.959907] follow_hugetlb_page: withhold 1 sec...
[ 17.960315] follow_hugetlb_page: withhold 1 sec...done
[ 17.960550] ------------[ cut here ]------------
[ 17.960742] DEBUG_LOCKS_WARN_ON(1)
[ 17.960756] WARNING: CPU: 3 PID: 542 at kernel/locking/lockdep.c:231 __lock_acquire+0x955/0x1fa0
[ 17.961264] Modules linked in:
[ 17.961394] CPU: 3 PID: 542 Comm: hugetlb-pmd-sha Not tainted 6.1.0-rc4-peterx+ #46
[ 17.961704] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
[ 17.962266] RIP: 0010:__lock_acquire+0x955/0x1fa0
[ 17.962516] Code: c0 0f 84 5f fe ff ff 44 8b 1d 0f 9a 29 02 45 85 db 0f 85 4f fe ff ff 48 c7 c6 75 50 83 82 48 c7 c7 1b 4b 7d 82 e8 d3 22 d8 00 <0f> 0b 31 c0 4c 8b 54 24 08 4c 8b 04 24 e9
[ 17.963494] RSP: 0018:ffffc90000e4fba8 EFLAGS: 00010096
[ 17.963704] RAX: 0000000000000016 RBX: fffffffffd3925a8 RCX: 0000000000000000
[ 17.963989] RDX: 0000000000000002 RSI: ffffffff82863ccf RDI: 00000000ffffffff
[ 17.964276] RBP: 0000000000000000 R08: 0000000000000000 R09: ffffc90000e4fa58
[ 17.964557] R10: 0000000000000003 R11: ffffffff83162688 R12: 0000000000000000
[ 17.964839] R13: 0000000000000001 R14: ffff888105eac748 R15: 0000000000000001
[ 17.965123] FS: 00007f17c0a00640(0000) GS:ffff888277cc0000(0000) knlGS:0000000000000000
[ 17.965443] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 17.965672] CR2: 00007f17c09ffef8 CR3: 000000010c87a005 CR4: 0000000000770ee0
[ 17.965956] PKRU: 55555554
[ 17.966068] Call Trace:
[ 17.966172] <TASK>
[ 17.966268] ? tick_nohz_tick_stopped+0x12/0x30
[ 17.966455] lock_acquire+0xbf/0x2b0
[ 17.966603] ? follow_hugetlb_page.cold+0x75/0x5c4
[ 17.966799] ? _printk+0x48/0x4e
[ 17.966934] _raw_spin_lock+0x2f/0x40
[ 17.967087] ? follow_hugetlb_page.cold+0x75/0x5c4
[ 17.967285] follow_hugetlb_page.cold+0x75/0x5c4
[ 17.967473] __get_user_pages+0xbb/0x620
[ 17.967635] faultin_vma_page_range+0x9a/0x100
[ 17.967817] madvise_vma_behavior+0x3c0/0xbd0
[ 17.967998] ? mas_prev+0x11/0x290
[ 17.968141] ? find_vma_prev+0x5e/0xa0
[ 17.968304] ? madvise_vma_anon_name+0x70/0x70
[ 17.968486] madvise_walk_vmas+0xa9/0x120
[ 17.968650] do_madvise.part.0+0xfa/0x270
[ 17.968813] __x64_sys_madvise+0x5a/0x70
[ 17.968974] do_syscall_64+0x37/0x90
[ 17.969123] entry_SYSCALL_64_after_hwframe+0x63/0xcd
[ 17.969329] RIP: 0033:0x7f1840f0efdb
[ 17.969477] Code: c3 66 0f 1f 44 00 00 48 8b 15 39 6e 0e 00 f7 d8 64 89 02 b8 ff ff ff ff eb bc 0f 1f 44 00 00 f3 0f 1e fa b8 1c 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 0d 68
[ 17.970205] RSP: 002b:00007f17c09ffe38 EFLAGS: 00000202 ORIG_RAX: 000000000000001c
[ 17.970504] RAX: ffffffffffffffda RBX: 00007f17c0a00640 RCX: 00007f1840f0efdb
[ 17.970786] RDX: 0000000000000017 RSI: 0000000000200000 RDI: 00007f1800000000
[ 17.971068] RBP: 00007f17c09ffe50 R08: 0000000000000000 R09: 00007ffd3954164f
[ 17.971353] R10: 00007f1840e10348 R11: 0000000000000202 R12: ffffffffffffff80
[ 17.971709] R13: 0000000000000000 R14: 00007ffd39541550 R15: 00007f17c0200000
[ 17.972083] </TASK>
[ 17.972199] irq event stamp: 2353
[ 17.972372] hardirqs last enabled at (2353): [<ffffffff8117fe4e>] __up_console_sem+0x5e/0x70
[ 17.972869] hardirqs last disabled at (2352): [<ffffffff8117fe33>] __up_console_sem+0x43/0x70
[ 17.973365] softirqs last enabled at (2330): [<ffffffff810f763d>] __irq_exit_rcu+0xed/0x160
[ 17.973857] softirqs last disabled at (2323): [<ffffffff810f763d>] __irq_exit_rcu+0xed/0x160
[ 17.974341] ---[ end trace 0000000000000000 ]---
[ 17.974614] BUG: kernel NULL pointer dereference, address: 00000000000000b8
[ 17.975012] #PF: supervisor read access in kernel mode
[ 17.975314] #PF: error_code(0x0000) - not-present page
[ 17.975615] PGD 103f7b067 P4D 103f7b067 PUD 106cd7067 PMD 0
[ 17.975943] Oops: 0000 [#1] PREEMPT SMP NOPTI
[ 17.976197] CPU: 3 PID: 542 Comm: hugetlb-pmd-sha Tainted: G W 6.1.0-rc4-peterx+ #46
[ 17.976712] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
[ 17.977370] RIP: 0010:__lock_acquire+0x190/0x1fa0
[ 17.977655] Code: 98 00 00 00 41 89 46 24 81 e2 ff 1f 00 00 48 0f a3 15 e4 ba dd 02 0f 83 ff 05 00 00 48 8d 04 52 48 c1 e0 06 48 05 c0 d2 f4 83 <44> 0f b6 a0 b8 00 00 00 41 0f b7 46 20 6f
[ 17.979170] RSP: 0018:ffffc90000e4fba8 EFLAGS: 00010046
[ 17.979787] RAX: 0000000000000000 RBX: fffffffffd3925a8 RCX: 0000000000000000
[ 17.980838] RDX: 0000000000000002 RSI: ffffffff82863ccf RDI: 00000000ffffffff
[ 17.982048] RBP: 0000000000000000 R08: ffff888105eac720 R09: ffffc90000e4fa58
[ 17.982892] R10: ffff888105eab900 R11: ffffffff83162688 R12: 0000000000000000
[ 17.983771] R13: 0000000000000001 R14: ffff888105eac748 R15: 0000000000000001
[ 17.984815] FS: 00007f17c0a00640(0000) GS:ffff888277cc0000(0000) knlGS:0000000000000000
[ 17.985924] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 17.986265] CR2: 00000000000000b8 CR3: 000000010c87a005 CR4: 0000000000770ee0
[ 17.986674] PKRU: 55555554
[ 17.986832] Call Trace:
[ 17.987012] <TASK>
[ 17.987266] ? tick_nohz_tick_stopped+0x12/0x30
[ 17.987770] lock_acquire+0xbf/0x2b0
[ 17.988118] ? follow_hugetlb_page.cold+0x75/0x5c4
[ 17.988575] ? _printk+0x48/0x4e
[ 17.988889] _raw_spin_lock+0x2f/0x40
[ 17.989243] ? follow_hugetlb_page.cold+0x75/0x5c4
[ 17.989687] follow_hugetlb_page.cold+0x75/0x5c4
[ 17.990119] __get_user_pages+0xbb/0x620
[ 17.990500] faultin_vma_page_range+0x9a/0x100
[ 17.990928] madvise_vma_behavior+0x3c0/0xbd0
[ 17.991354] ? mas_prev+0x11/0x290
[ 17.991678] ? find_vma_prev+0x5e/0xa0
[ 17.992024] ? madvise_vma_anon_name+0x70/0x70
[ 17.992421] madvise_walk_vmas+0xa9/0x120
[ 17.992793] do_madvise.part.0+0xfa/0x270
[ 17.993166] __x64_sys_madvise+0x5a/0x70
[ 17.993539] do_syscall_64+0x37/0x90
[ 17.993879] entry_SYSCALL_64_after_hwframe+0x63/0xcd
======8<=======

Resolution
==========

This patchset protects all the huge_pte_offset() callers to also take the
vma lock properly.

Patch Layout
============

Patch 1-2: cleanup, or dependency of the follow up patches
Patch 3: before fixing, document huge_pte_offset() on lock required
Patch 4-8: each patch resolves one possible race condition
Patch 9: introduce hugetlb_walk() to replace huge_pte_offset()
Patch 10: document why page_vma_mapped_walk() is safe

Tests
=====

The series is verified with the above reproducer so the race cannot
trigger anymore. It also passes all hugetlb kselftests.

Comments welcomed, thanks.

Peter Xu (10):
mm/hugetlb: Let vma_offset_start() to return start
mm/hugetlb: Don't wait for migration entry during follow page
mm/hugetlb: Document huge_pte_offset usage
mm/hugetlb: Move swap entry handling into vma lock when faulted
mm/hugetlb: Make userfaultfd_huge_must_wait() safe to pmd unshare
mm/hugetlb: Make hugetlb_follow_page_mask() safe to pmd unshare
mm/hugetlb: Make follow_hugetlb_page() safe to pmd unshare
mm/hugetlb: Make walk_hugetlb_range() safe to pmd unshare
mm/hugetlb: Introduce hugetlb_walk()
mm/hugetlb: Document why page_vma_mapped_walk() is safe to walk

arch/s390/mm/gmap.c | 2 +
fs/hugetlbfs/inode.c | 28 ++++++-------
fs/proc/task_mmu.c | 2 +
fs/userfaultfd.c | 24 +++++++----
include/linux/hugetlb.h | 71 +++++++++++++++++++++++++++++++
include/linux/pagewalk.h | 11 ++++-
include/linux/swapops.h | 6 ++-
mm/hmm.c | 15 ++++++-
mm/hugetlb.c | 91 +++++++++++++++++-----------------------
mm/migrate.c | 25 +++++++++--
mm/page_vma_mapped.c | 12 ++++--
mm/pagewalk.c | 6 +--
12 files changed, 204 insertions(+), 89 deletions(-)

--
2.37.3