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Wed, 19 Feb 2025 10:40:31 -0800 (PST) MIME-Version: 1.0 References: <20250219112519.92853-1-21cnbao@gmail.com> In-Reply-To: <20250219112519.92853-1-21cnbao@gmail.com> From: Lokesh Gidra Date: Wed, 19 Feb 2025 10:40:19 -0800 X-Gm-Features: AWEUYZn0eb1WdvDVflIr5RFfBe6ebJUvO0pCSFFXvZBQ6PCLVBN2j74RX1OKjWc Message-ID: Subject: Re: [PATCH RFC] mm: Fix kernel BUG when userfaultfd_move encounters swapcache To: Barry Song <21cnbao@gmail.com> Cc: linux-mm@kvack.org, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, zhengtangquan@oppo.com, Barry Song , Andrea Arcangeli , Suren Baghdasaryan , Al Viro , Axel Rasmussen , Brian Geffon , Christian Brauner , David Hildenbrand , Hugh Dickins , Jann Horn , Kalesh Singh , "Liam R . Howlett" , Matthew Wilcox , Michal Hocko , Mike Rapoport , Nicolas Geoffray , Peter Xu , Ryan Roberts , Shuah Khan , ZhangPeng Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 854D11C0020 X-Stat-Signature: q6gr1irmmjyhzj1r3ock1fxneh384hrm X-HE-Tag: 1739990432-576738 X-HE-Meta: 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 +quiNjoH 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Feb 19, 2025 at 3:25=E2=80=AFAM Barry Song <21cnbao@gmail.com> wrot= e: > > From: Barry Song > > userfaultfd_move() checks whether the PTE entry is present or a > swap entry. > > - If the PTE entry is present, move_present_pte() handles folio > migration by setting: > > src_folio->index =3D linear_page_index(dst_vma, dst_addr); > > - If the PTE entry is a swap entry, move_swap_pte() simply copies > the PTE to the new dst_addr. > > This approach is incorrect because even if the PTE is a swap > entry, it can still reference a folio that remains in the swap > cache. > > If do_swap_page() is triggered, it may locate the folio in the > swap cache. However, during add_rmap operations, a kernel panic > can occur due to: > page_pgoff(folio, page) !=3D linear_page_index(vma, address) > > $./a.out > /dev/null > [ 13.336953] page: refcount:6 mapcount:1 mapping:00000000f43db19c index= :0xffffaf150 pfn:0x4667c > [ 13.337520] head: order:2 mapcount:1 entire_mapcount:0 nr_pages_mapped= :1 pincount:0 > [ 13.337716] memcg:ffff00000405f000 > [ 13.337849] anon flags: 0x3fffc0000020459(locked|uptodate|dirty|owner_= priv_1|head|swapbacked|node=3D0|zone=3D0|lastcpupid=3D0xffff) > [ 13.338630] raw: 03fffc0000020459 ffff80008507b538 ffff80008507b538 ff= ff000006260361 > [ 13.338831] raw: 0000000ffffaf150 0000000000004000 0000000600000000 ff= ff00000405f000 > [ 13.339031] head: 03fffc0000020459 ffff80008507b538 ffff80008507b538 f= fff000006260361 > [ 13.339204] head: 0000000ffffaf150 0000000000004000 0000000600000000 f= fff00000405f000 > [ 13.339375] head: 03fffc0000000202 fffffdffc0199f01 ffffffff00000000 0= 000000000000001 > [ 13.339546] head: 0000000000000004 0000000000000000 00000000ffffffff 0= 000000000000000 > [ 13.339736] page dumped because: VM_BUG_ON_PAGE(page_pgoff(folio, page= ) !=3D linear_page_index(vma, address)) > [ 13.340190] ------------[ cut here ]------------ > [ 13.340316] kernel BUG at mm/rmap.c:1380! > [ 13.340683] Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT = SMP > [ 13.340969] Modules linked in: > [ 13.341257] CPU: 1 UID: 0 PID: 107 Comm: a.out Not tainted 6.14.0-rc3-= gcf42737e247a-dirty #299 > [ 13.341470] Hardware name: linux,dummy-virt (DT) > [ 13.341671] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYP= E=3D--) > [ 13.341815] pc : __page_check_anon_rmap+0xa0/0xb0 > [ 13.341920] lr : __page_check_anon_rmap+0xa0/0xb0 > [ 13.342018] sp : ffff80008752bb20 > [ 13.342093] x29: ffff80008752bb20 x28: fffffdffc0199f00 x27: 000000000= 0000001 > [ 13.342404] x26: 0000000000000000 x25: 0000000000000001 x24: 000000000= 0000001 > [ 13.342575] x23: 0000ffffaf0d0000 x22: 0000ffffaf0d0000 x21: fffffdffc= 0199f00 > [ 13.342731] x20: fffffdffc0199f00 x19: ffff000006210700 x18: 00000000f= fffffff > [ 13.342881] x17: 6c203d2120296567 x16: 6170202c6f696c6f x15: 662866666= f67705f > [ 13.343033] x14: 6567617028454741 x13: 2929737365726464 x12: ffff80008= 3728ab0 > [ 13.343183] x11: ffff800082996bf8 x10: 0000000000000fd7 x9 : ffff80008= 011bc40 > [ 13.343351] x8 : 0000000000017fe8 x7 : 00000000fffff000 x6 : ffff80008= 29eebf8 > [ 13.343498] x5 : c0000000fffff000 x4 : 0000000000000000 x3 : 000000000= 0000000 > [ 13.343645] x2 : 0000000000000000 x1 : ffff0000062db980 x0 : 000000000= 000005f > [ 13.343876] Call trace: > [ 13.344045] __page_check_anon_rmap+0xa0/0xb0 (P) > [ 13.344234] folio_add_anon_rmap_ptes+0x22c/0x320 > [ 13.344333] do_swap_page+0x1060/0x1400 > [ 13.344417] __handle_mm_fault+0x61c/0xbc8 > [ 13.344504] handle_mm_fault+0xd8/0x2e8 > [ 13.344586] do_page_fault+0x20c/0x770 > [ 13.344673] do_translation_fault+0xb4/0xf0 > [ 13.344759] do_mem_abort+0x48/0xa0 > [ 13.344842] el0_da+0x58/0x130 > [ 13.344914] el0t_64_sync_handler+0xc4/0x138 > [ 13.345002] el0t_64_sync+0x1ac/0x1b0 > [ 13.345208] Code: aa1503e0 f000f801 910f6021 97ff5779 (d4210000) > [ 13.345504] ---[ end trace 0000000000000000 ]--- > [ 13.345715] note: a.out[107] exited with irqs disabled > [ 13.345954] note: a.out[107] exited with preempt_count 2 > > Fully fixing it would be quite complex, requiring similar handling > of folios as done in move_present_pte. For now, a quick solution > is to return -EBUSY. > I'd like to see others' opinions on whether a full fix is worth > pursuing. > Thanks a lot for finding this. As a user of MOVE ioctl (in Android GC) I strongly urge you to fix this properly. Because this is not going to be a rare occurrence in the case of Android. And when -EBUSY is returned, all that userspace can do is touch the page, which also does not guarantee that a subsequent retry of the ioctl will succeed. > For anyone interested in reproducing it, the a.out test program is > as below, > > #define _GNU_SOURCE > #include > #include > #include > #include > #include > #include > #include > #include > #include > #include > #include > #include > > #define PAGE_SIZE 4096 > #define REGION_SIZE (512 * 1024) > > #ifndef UFFDIO_MOVE > struct uffdio_move { > __u64 dst; > __u64 src; > __u64 len; > #define UFFDIO_MOVE_MODE_DONTWAKE ((__u64)1<<0) > #define UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES ((__u64)1<<1) > __u64 mode; > __s64 move; > }; > #define _UFFDIO_MOVE (0x05) > #define UFFDIO_MOVE _IOWR(UFFDIO, _UFFDIO_MOVE, struct uffdio_move) > #endif > > void *src, *dst; > int uffd; > > void *madvise_thread(void *arg) { > if (madvise(src, REGION_SIZE, MADV_PAGEOUT) =3D=3D -1) { > perror("madvise MADV_PAGEOUT"); > } > return NULL; > } > > void *fault_handler_thread(void *arg) { > struct uffd_msg msg; > struct uffdio_move move; > struct pollfd pollfd =3D { .fd =3D uffd, .events =3D POLLIN }; > > pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL); > pthread_setcanceltype(PTHREAD_CANCEL_DEFERRED, NULL); > > while (1) { > if (poll(&pollfd, 1, -1) =3D=3D -1) { > perror("poll"); > exit(EXIT_FAILURE); > } > > if (read(uffd, &msg, sizeof(msg)) <=3D 0) { > perror("read"); > exit(EXIT_FAILURE); > } > > if (msg.event !=3D UFFD_EVENT_PAGEFAULT) { > fprintf(stderr, "Unexpected event\n"); > exit(EXIT_FAILURE); > } > > move.src =3D (unsigned long)src + (msg.arg.pagefault.address - (= unsigned long)dst); > move.dst =3D msg.arg.pagefault.address & ~(PAGE_SIZE - 1); > move.len =3D PAGE_SIZE; > move.mode =3D 0; > > if (ioctl(uffd, UFFDIO_MOVE, &move) =3D=3D -1) { > perror("UFFDIO_MOVE"); > exit(EXIT_FAILURE); > } > } > return NULL; > } > > int main() { > again: > pthread_t thr, madv_thr; > struct uffdio_api uffdio_api =3D { .api =3D UFFD_API, .features =3D = 0 }; > struct uffdio_register uffdio_register; > > src =3D mmap(NULL, REGION_SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE = | MAP_ANONYMOUS, -1, 0); > if (src =3D=3D MAP_FAILED) { > perror("mmap src"); > exit(EXIT_FAILURE); > } > memset(src, 1, REGION_SIZE); > > dst =3D mmap(NULL, REGION_SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE = | MAP_ANONYMOUS, -1, 0); > if (dst =3D=3D MAP_FAILED) { > perror("mmap dst"); > exit(EXIT_FAILURE); > } > > uffd =3D syscall(SYS_userfaultfd, O_CLOEXEC | O_NONBLOCK); > if (uffd =3D=3D -1) { > perror("userfaultfd"); > exit(EXIT_FAILURE); > } > > if (ioctl(uffd, UFFDIO_API, &uffdio_api) =3D=3D -1) { > perror("UFFDIO_API"); > exit(EXIT_FAILURE); > } > > uffdio_register.range.start =3D (unsigned long)dst; > uffdio_register.range.len =3D REGION_SIZE; > uffdio_register.mode =3D UFFDIO_REGISTER_MODE_MISSING; > > if (ioctl(uffd, UFFDIO_REGISTER, &uffdio_register) =3D=3D -1) { > perror("UFFDIO_REGISTER"); > exit(EXIT_FAILURE); > } > > if (pthread_create(&madv_thr, NULL, madvise_thread, NULL) !=3D 0) { > perror("pthread_create madvise_thread"); > exit(EXIT_FAILURE); > } > > if (pthread_create(&thr, NULL, fault_handler_thread, NULL) !=3D 0) { > perror("pthread_create fault_handler_thread"); > exit(EXIT_FAILURE); > } > > for (size_t i =3D 0; i < REGION_SIZE; i +=3D PAGE_SIZE) { > char val =3D ((char *)dst)[i]; > printf("Accessing dst at offset %zu, value: %d\n", i, val); > } > > pthread_join(madv_thr, NULL); > pthread_cancel(thr); > pthread_join(thr, NULL); > > munmap(src, REGION_SIZE); > munmap(dst, REGION_SIZE); > close(uffd); > goto again; > return 0; > } > > As long as you enable mTHP (which likely increases the residency > time of swapcache), you can reproduce the issue within a few > seconds. But I guess the same race condition also exists with > small folios. > > Fixes: adef440691bab ("userfaultfd: UFFDIO_MOVE uABI") > Cc: Andrea Arcangeli > Cc: Suren Baghdasaryan > Cc: Al Viro > Cc: Axel Rasmussen > Cc: Brian Geffon > Cc: Christian Brauner > Cc: David Hildenbrand > Cc: Hugh Dickins > Cc: Jann Horn > Cc: Kalesh Singh > Cc: Liam R. Howlett > Cc: Lokesh Gidra > Cc: Matthew Wilcox (Oracle) > Cc: Michal Hocko > Cc: Mike Rapoport (IBM) > Cc: Nicolas Geoffray > Cc: Peter Xu > Cc: Ryan Roberts > Cc: Shuah Khan > Cc: ZhangPeng > Signed-off-by: Barry Song > --- > mm/userfaultfd.c | 11 +++++++++++ > 1 file changed, 11 insertions(+) > > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c > index 867898c4e30b..34cf1c8c725d 100644 > --- a/mm/userfaultfd.c > +++ b/mm/userfaultfd.c > @@ -18,6 +18,7 @@ > #include > #include > #include "internal.h" > +#include "swap.h" > > static __always_inline > bool validate_dst_vma(struct vm_area_struct *dst_vma, unsigned long dst_= end) > @@ -1079,9 +1080,19 @@ static int move_swap_pte(struct mm_struct *mm, > pmd_t *dst_pmd, pmd_t dst_pmdval, > spinlock_t *dst_ptl, spinlock_t *src_ptl) > { > + struct folio *folio; > + swp_entry_t entry; > + > if (!pte_swp_exclusive(orig_src_pte)) > return -EBUSY; > > + entry =3D pte_to_swp_entry(orig_src_pte); > + folio =3D filemap_get_folio(swap_address_space(entry), swap_cache= _index(entry)); > + if (!IS_ERR(folio)) { > + folio_put(folio); > + return -EBUSY; > + } > + > double_pt_lock(dst_ptl, src_ptl); > > if (!is_pte_pages_stable(dst_pte, src_pte, orig_dst_pte, orig_src= _pte, > -- > 2.39.3 (Apple Git-146) >