Re: [PATCH] mm/mmap/vma_merge: always check invariants

[Lorenzo Stoakes]

On Wed, May 10, 2023 at 03:15:51PM +0100, Mark Rutland wrote:
> Hi,
>
> On Sun, Apr 30, 2023 at 09:19:17PM +0100, Lorenzo Stoakes wrote:
> > We may still have inconsistent input parameters even if we choose not to
> > merge and the vma_merge() invariant checks are useful for checking this
> > with no production runtime cost (these are only relevant when
> > CONFIG_DEBUG_VM is specified).
> >
> > Therefore, perform these checks regardless of whether we merge.
> >
> > This is relevant, as a recent issue (addressed in commit "mm/mempolicy:
> > Correctly update prev when policy is equal on mbind") in the mbind logic
> > was only picked up in the 6.2.y stable branch where these assertions are
> > performed prior to determining mergeability.
> >
> > Had this remained the same in mainline this issue may have been picked up
> > faster, so moving forward let's always check them.
> >
> > Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
> > ---
> >  mm/mmap.c | 10 +++++-----
> >  1 file changed, 5 insertions(+), 5 deletions(-)
> >
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 5522130ae606..13678edaa22c 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -960,17 +960,17 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> >  		merge_next = true;
> >  	}
> >
> > +	/* Verify some invariant that must be enforced by the caller. */
> > +	VM_WARN_ON(prev && addr <= prev->vm_start);
> > +	VM_WARN_ON(curr && (addr != curr->vm_start || end > curr->vm_end));
> > +	VM_WARN_ON(addr >= end);
> > +
>
> I'm seeing this fire a lot when fuzzing v6.4-rc1 on arm64 using Syzkaller.
>

Thanks, from the line I suspect addr != curr->vm_start, but need to look
into the repro, at lsf/mm so a bit time lagged :)

Am looking into it!

> The splat looks like:
>
> | Syzkaller hit 'WARNING in vma_merge' bug.
> |
> | ------------[ cut here ]------------
> | WARNING: CPU: 0 PID: 193 at mm/mmap.c:965 vma_merge+0x21c/0x1158 mm/mmap.c:965
> | CPU: 0 PID: 193 Comm: syz-executor105 Not tainted 6.4.0-rc1-00001-g7d54d3135001 #1
> | Hardware name: linux,dummy-virt (DT)
> | pstate: 80400005 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> | pc : vma_merge+0x21c/0x1158 mm/mmap.c:965
> | lr : vma_merge+0x21c/0x1158 mm/mmap.c:965
> | sp : ffff800018ec7970
> | x29: ffff800018ec7970 x28: 0000000020000000 x27: 0000000000000000
> | x26: 0000000000000000 x25: 1ffff000031d8f42 x24: ffff000010d58000
> | x23: 0000000000000000 x22: ffff000017acc9b0 x21: 0000000020ffd000
> | x20: 0000000020ffb000 x19: ffff000017acc8b8 x18: 0000000000000005
> | x17: 0000000000000000 x16: 0000000000000000 x15: 1fffe00002f27494
> | x14: 0000000000000000 x13: 000000009a8feb3a x12: ffff700002ddc77d
> | x11: 1ffff00002ddc77c x10: ffff700002ddc77c x9 : dfff800000000000
> | x8 : ffff800016ee3be3 x7 : 0000000000000000 x6 : 0000000000000000
> | x5 : ffff000017939b00 x4 : ffff800010c4a000 x3 : ffff800008000000
> | x2 : 0000000000000000 x1 : ffff000017939b00 x0 : 0000000000000000
> | Call trace:
> |  vma_merge+0x21c/0x1158 mm/mmap.c:965
> |  userfaultfd_register fs/userfaultfd.c:1485 [inline]
> |  userfaultfd_ioctl+0x378c/0x4240 fs/userfaultfd.c:2050
> |  vfs_ioctl fs/ioctl.c:51 [inline]
> |  __do_sys_ioctl fs/ioctl.c:870 [inline]
> |  __se_sys_ioctl fs/ioctl.c:856 [inline]
> |  __arm64_sys_ioctl+0x184/0x218 fs/ioctl.c:856
> |  __invoke_syscall arch/arm64/kernel/syscall.c:38 [inline]
> |  invoke_syscall+0x8c/0x2d8 arch/arm64/kernel/syscall.c:52
> |  el0_svc_common.constprop.0+0xf4/0x300 arch/arm64/kernel/syscall.c:142
> |  do_el0_svc+0x6c/0x180 arch/arm64/kernel/syscall.c:193
> |  el0_svc+0x4c/0x110 arch/arm64/kernel/entry-common.c:637
> |  el0t_64_sync_handler+0xf4/0x120 arch/arm64/kernel/entry-common.c:655
> |  el0t_64_sync+0x190/0x198 arch/arm64/kernel/entry.S:591
> | irq event stamp: 2212
> | hardirqs last  enabled at (2211): [<ffff80000805d03c>] local_daif_restore arch/arm64/include/asm/daifflags.h:75 [inline]
> | hardirqs last  enabled at (2211): [<ffff80000805d03c>] el0_svc_common.constprop.0+0xac/0x300 arch/arm64/kernel/syscall.c:107
> | hardirqs last disabled at (2212): [<ffff80000e67c40c>] el1_dbg+0x24/0xa0 arch/arm64/kernel/entry-common.c:405
> | softirqs last  enabled at (2190): [<ffff800008021718>] softirq_handle_end kernel/softirq.c:414 [inline]
> | softirqs last  enabled at (2190): [<ffff800008021718>] __do_softirq+0x8e8/0xe50 kernel/softirq.c:600
> | softirqs last disabled at (2183): [<ffff80000802ad9c>] ____do_softirq+0x1c/0x30 arch/arm64/kernel/irq.c:80
> | ---[ end trace 0000000000000000 ]---
>
> I can reproduce that reliably with the below:
>
> | #include <linux/userfaultfd.h>
> | #include <sys/ioctl.h>
> | #include <sys/mman.h>
> | #include <sys/syscall.h>
> | #include <sys/types.h>
> | #include <unistd.h>
> |
> | int main(int argc, char *argv[])
> | {
> |         int uffd;
> |         void *addr;
> |
> |         struct uffdio_api uffdio_api;
> |         struct uffdio_register uffdio_register;
> |
> |         uffd = syscall(__NR_userfaultfd, 0x801ul);
> |
> |         uffdio_api.api = UFFD_API;
> |         uffdio_api.features = 0;
> |         ioctl(uffd, UFFDIO_API, &uffdio_api);
> |
> |         addr = mmap(NULL, 0x1000000ul, PROT_READ | PROT_WRITE,
> |                     MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
> |
> |         uffdio_register.range.start = (unsigned long)addr + 0x10000;
> |         uffdio_register.range.len = 0x2000;
> |         uffdio_register.mode = UFFDIO_REGISTER_MODE_MISSING;
> |         ioctl(uffd, UFFDIO_REGISTER, &uffdio_register);
> |
> |         return 0;
> | }
>
> ... which is cleaned up from the orginial Syzkaller reproducer:
>
> | Syzkaller reproducer:
> | # {Threaded:false Repeat:false RepeatTimes:0 Procs:1 Slowdown:1 Sandbox: SandboxArg:0 Leak:false NetInjection:false NetDevices:false NetReset:false Cgroups:false BinfmtMisc:false CloseFDs:false KCSAN:false DevlinkPCI:false NicVF:false USB:false VhciInjection:false Wifi:false IEEE802154:false Sysctl:false UseTmpDir:false HandleSegv:false Repro:false Trace:false LegacyOptions:{Collide:false Fault:false FaultCall:0 FaultNth:0}}
> | r0 = userfaultfd(0x801)
> | r1 = dup(r0)
> | ioctl$UFFDIO_API(r1, 0xc018aa3f, &(0x7f0000000000))
> | ioctl$UFFDIO_REGISTER(r1, 0xc020aa00, &(0x7f00000001c0)={{&(0x7f0000ffb000/0x2000)=nil, 0x2000}, 0x1})
> |
> |
> | C reproducer:
> | // autogenerated by syzkaller (https://github.com/google/syzkaller)
> |
> | #define _GNU_SOURCE
> |
> | #include <endian.h>
> | #include <stdint.h>
> | #include <stdio.h>
> | #include <stdlib.h>
> | #include <string.h>
> | #include <sys/syscall.h>
> | #include <sys/types.h>
> | #include <unistd.h>
> |
> | #ifndef __NR_dup
> | #define __NR_dup 23
> | #endif
> | #ifndef __NR_ioctl
> | #define __NR_ioctl 29
> | #endif
> | #ifndef __NR_mmap
> | #define __NR_mmap 222
> | #endif
> | #ifndef __NR_userfaultfd
> | #define __NR_userfaultfd 282
> | #endif
> |
> | uint64_t r[2] = {0xffffffffffffffff, 0xffffffffffffffff};
> |
> | int main(void)
> | {
> | 		syscall(__NR_mmap, 0x1ffff000ul, 0x1000ul, 0ul, 0x32ul, -1, 0ul);
> | 	syscall(__NR_mmap, 0x20000000ul, 0x1000000ul, 7ul, 0x32ul, -1, 0ul);
> | 	syscall(__NR_mmap, 0x21000000ul, 0x1000ul, 0ul, 0x32ul, -1, 0ul);
> | 				intptr_t res = 0;
> | 	res = syscall(__NR_userfaultfd, 0x801ul);
> | 	if (res != -1)
> | 		r[0] = res;
> | 	res = syscall(__NR_dup, r[0]);
> | 	if (res != -1)
> | 		r[1] = res;
> | *(uint64_t*)0x20000000 = 0xaa;
> | *(uint64_t*)0x20000008 = 0;
> | *(uint64_t*)0x20000010 = 0;
> | 	syscall(__NR_ioctl, r[1], 0xc018aa3f, 0x20000000ul);
> | *(uint64_t*)0x200001c0 = 0x20ffb000;
> | *(uint64_t*)0x200001c8 = 0x2000;
> | *(uint64_t*)0x200001d0 = 1;
> | *(uint64_t*)0x200001d8 = 0;
> | 	syscall(__NR_ioctl, r[1], 0xc020aa00, 0x200001c0ul);
> | 	return 0;
> | }
>
> Thanks,
> Mark.