On Tue, Jun 28, 2022 at 03:59:26PM -0700, syzbot wrote: > Hello, > > syzbot found the following issue on: > > HEAD commit: 941e3e791269 Merge tag 'for_linus' of git://git.kernel.org.. > git tree: upstream > console+strace: https://syzkaller.appspot.com/x/log.txt?x=1670ded4080000 > kernel config: https://syzkaller.appspot.com/x/.config?x=833001d0819ddbc9 > dashboard link: https://syzkaller.appspot.com/bug?extid=9bd2b7adbd34b30b87e4 > compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2 > syz repro: https://syzkaller.appspot.com/x/repro.syz?x=140f9ba8080000 > C reproducer: https://syzkaller.appspot.com/x/repro.c?x=15495188080000 > > IMPORTANT: if you fix the issue, please add the following tag to the commit: > Reported-by: syzbot+9bd2b7adbd34b30b87e4@xxxxxxxxxxxxxxxxxxxxxxxxx > > BUG: unable to handle page fault for address: ffff888021f7e005 > #PF: supervisor write access in kernel mode > #PF: error_code(0x0002) - not-present page > PGD 11401067 P4D 11401067 PUD 11402067 PMD 21f7d063 PTE 800fffffde081060 > Oops: 0002 [#1] PREEMPT SMP KASAN > CPU: 0 PID: 3761 Comm: syz-executor281 Not tainted 5.19.0-rc4-syzkaller-00014-g941e3e791269 #0 > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 > RIP: 0010:memset_erms+0x9/0x10 arch/x86/lib/memset_64.S:64 > Code: c1 e9 03 40 0f b6 f6 48 b8 01 01 01 01 01 01 01 01 48 0f af c6 f3 48 ab 89 d1 f3 aa 4c 89 c8 c3 90 49 89 f9 40 88 f0 48 89 d1 <f3> aa 4c 89 c8 c3 90 49 89 fa 40 0f b6 ce 48 b8 01 01 01 01 01 01 > RSP: 0018:ffffc9000329fa90 EFLAGS: 00010202 > RAX: 0000000000000000 RBX: 0000000000001000 RCX: 0000000000000ffb > RDX: 0000000000000ffb RSI: 0000000000000000 RDI: ffff888021f7e005 > RBP: ffffea000087df80 R08: 0000000000000001 R09: ffff888021f7e005 > R10: ffffed10043efdff R11: 0000000000000000 R12: 0000000000000005 > R13: 0000000000000000 R14: 0000000000001000 R15: 0000000000000ffb > FS: 00007fb29d8b2700(0000) GS:ffff8880b9a00000(0000) knlGS:0000000000000000 > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > CR2: ffff888021f7e005 CR3: 0000000026e7b000 CR4: 00000000003506f0 > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 > Call Trace: > <TASK> > zero_user_segments include/linux/highmem.h:272 [inline] > folio_zero_range include/linux/highmem.h:428 [inline] > truncate_inode_partial_folio+0x76a/0xdf0 mm/truncate.c:237 > truncate_inode_pages_range+0x83b/0x1530 mm/truncate.c:381 > truncate_inode_pages mm/truncate.c:452 [inline] > truncate_pagecache+0x63/0x90 mm/truncate.c:753 > simple_setattr+0xed/0x110 fs/libfs.c:535 > secretmem_setattr+0xae/0xf0 mm/secretmem.c:170 > notify_change+0xb8c/0x12b0 fs/attr.c:424 > do_truncate+0x13c/0x200 fs/open.c:65 > do_sys_ftruncate+0x536/0x730 fs/open.c:193 > do_syscall_x64 arch/x86/entry/common.c:50 [inline] > do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80 > entry_SYSCALL_64_after_hwframe+0x46/0xb0 > RIP: 0033:0x7fb29d900899 > Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 11 15 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48 > RSP: 002b:00007fb29d8b2318 EFLAGS: 00000246 ORIG_RAX: 000000000000004d > RAX: ffffffffffffffda RBX: 00007fb29d988408 RCX: 00007fb29d900899 > RDX: 00007fb29d900899 RSI: 0000000000000005 RDI: 0000000000000003 > RBP: 00007fb29d988400 R08: 0000000000000000 R09: 0000000000000000 > R10: 0000000000000000 R11: 0000000000000246 R12: 00007fb29d98840c > R13: 00007ffca01a23bf R14: 00007fb29d8b2400 R15: 0000000000022000 > </TASK> > Modules linked in: > CR2: ffff888021f7e005 > ---[ end trace 0000000000000000 ]--- I think this is a bug in memfd_secret. secretmem_setattr() can race with a page being faulted in by secretmem_fault(). Specifically, a page can be faulted in after secretmem_setattr() has set i_size but before it zeroes out the partial page past i_size. memfd_secret pages aren't mapped in the kernel direct map, so the crash occurs when the kernel tries to zero out the partial page. I don't know what the best solution is -- maybe a rw_semaphore protecting secretmem_fault() and secretmem_setattr()? Or perhaps secretmem_setattr() should avoid the call to truncate_setsize() by not using simple_setattr(), given that secretmem_setattr() only supports the size going from zero to nonzero. The following commit tried to fix a similar bug, but it wasn't enough: commit f9b141f93659e09a52e28791ccbaf69c273b8e92 Author: Axel Rasmussen <axelrasmussen@xxxxxxxxxx> Date: Thu Apr 14 19:13:31 2022 -0700 mm/secretmem: fix panic when growing a memfd_secret Here's a simplified reproducer. Note, for memfd_secret to be supported, the kernel config must contain CONFIG_SECRETMEM=y and the kernel command line must contain secretmem.enable=1. #include <pthread.h> #include <setjmp.h> #include <signal.h> #include <sys/mman.h> #include <sys/syscall.h> #include <unistd.h> static volatile int fd; static jmp_buf jump_buf; static void *truncate_thread(void *arg) { for (;;) ftruncate(fd, 1000); } static void handle_sigbus(int sig) { longjmp(jump_buf, 1); } int main(void) { struct sigaction act = { .sa_handler = handle_sigbus, .sa_flags = SA_NODEFER, }; pthread_t t; void *addr; sigaction(SIGBUS, &act, NULL); pthread_create(&t, NULL, truncate_thread, NULL); for (;;) { fd = syscall(__NR_memfd_secret, 0); addr = mmap(NULL, 8192, PROT_WRITE, MAP_SHARED, fd, 0); if (setjmp(jump_buf) == 0) *(unsigned int *)addr = 0; munmap(addr, 8192); close(fd); } }
