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Subjectuserfaultfd use-after-free
Hi,

Syzkaller found a use-after-free in the userfaultfd code, which is very
easy to trigger on x86_64 and arm64 in v4.13 and v4.14-rc1. I believe
this may go as far back as v4.10, when fork events were introduced.

While fuzzing I've been seeing some other intermittent memory corruption bugs
which I believe are related -- I only tracked this down to userfaultfd after
enabling both KASAN and DEBUG_LIST.

I've included an example log, a (hopefully legible) C reproducer, and Syzkaller
reproducer below. These can also be found on my kernel.org web space, along
with other logs, configs, etc:

https://www.kernel.org/pub/linux/kernel/people/mark/bugs/20170919-userfaultfd-uaf/

Thanks,
Mark.

arm64 log (v4.13)
----
[ 3.286772] ==================================================================
[ 3.288470] BUG: KASAN: use-after-free in __list_del_entry_valid+0x148/0x188
[ 3.290230] Read of size 8 at addr ffff80000af53b40 by task repro/1374
[ 3.291682]
[ 3.292099] CPU: 2 PID: 1374 Comm: repro Not tainted 4.13.0 #47
[ 3.293653] Hardware name: linux,dummy-virt (DT)
[ 3.294862] Call trace:
[ 3.295506] [<ffff20000808fd00>] dump_backtrace+0x0/0x420
[ 3.296887] [<ffff2000080903ec>] show_stack+0x14/0x20
[ 3.298173] [<ffff2000098c1424>] dump_stack+0xcc/0xf8
[ 3.299463] [<ffff2000083dc2c0>] print_address_description+0x60/0x250
[ 3.301101] [<ffff2000083dc7b0>] kasan_report+0x238/0x2f8
[ 3.302474] [<ffff2000083dc8e8>] __asan_report_load8_noabort+0x18/0x20
[ 3.304144] [<ffff2000088d74a0>] __list_del_entry_valid+0x148/0x188
[ 3.305734] [<ffff2000084dc5d8>] userfaultfd_event_wait_completion+0x278/0x568
[ 3.307567] [<ffff2000084e0f38>] dup_userfaultfd_complete+0x110/0x290
[ 3.309205] [<ffff200008114df4>] copy_process.isra.6.part.7+0x39b4/0x4768
[ 3.310920] [<ffff200008115f60>] _do_fork+0x120/0x590
[ 3.312209] [<ffff200008116498>] SyS_clone+0x18/0x20
[ 3.313471] [<ffff200008083f30>] el0_svc_naked+0x24/0x28
[ 3.314816]
[ 3.315212] The buggy address belongs to the page:
[ 3.316439] page:ffff7e00002bd4c0 count:0 mapcount:0 mapping: (null) index:0x0
[ 3.318456] flags: 0xfffc00000000000()
[ 3.319208] raw: 0fffc00000000000 0000000000000000 0000000000000000 00000000ffffffff
[ 3.321177] raw: dead000000000100 dead000000000200 0000000000000000 0000000000000000
[ 3.323125] page dumped because: kasan: bad access detected
[ 3.324542]
[ 3.324938] Memory state around the buggy address:
[ 3.326155] ffff80000af53a00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
[ 3.327983] ffff80000af53a80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
[ 3.329808] >ffff80000af53b00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
[ 3.331635] ^
[ 3.332980] ffff80000af53b80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
[ 3.334801] ffff80000af53c00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
[ 3.336627] ==================================================================
[ 3.338447] Disabling lock debugging due to kernel taint
[ 3.339915] Kernel panic - not syncing: panic_on_warn set ...
[ 3.339915]
[ 3.341081] CPU: 2 PID: 1374 Comm: repro Tainted: G B 4.13.0 #47
[ 3.342884] Hardware name: linux,dummy-virt (DT)
[ 3.344062] Call trace:
[ 3.344698] [<ffff20000808fd00>] dump_backtrace+0x0/0x420
[ 3.346066] [<ffff2000080903ec>] show_stack+0x14/0x20
[ 3.347346] [<ffff2000098c1424>] dump_stack+0xcc/0xf8
[ 3.348637] [<ffff2000081179c4>] panic+0x1e4/0x358
[ 3.349855] [<ffff2000083dc230>] kasan_save_enable_multi_shot+0x0/0x30
[ 3.351504] [<ffff2000083dc66c>] kasan_report+0xf4/0x2f8
[ 3.352860] [<ffff2000083dc8e8>] __asan_report_load8_noabort+0x18/0x20
[ 3.354509] [<ffff2000088d74a0>] __list_del_entry_valid+0x148/0x188
[ 3.356101] [<ffff2000084dc5d8>] userfaultfd_event_wait_completion+0x278/0x568
[ 3.357920] [<ffff2000084e0f38>] dup_userfaultfd_complete+0x110/0x290
[ 3.359553] [<ffff200008114df4>] copy_process.isra.6.part.7+0x39b4/0x4768
[ 3.361267] [<ffff200008115f60>] _do_fork+0x120/0x590
[ 3.362549] [<ffff200008116498>] SyS_clone+0x18/0x20
[ 3.363815] [<ffff200008083f30>] el0_svc_naked+0x24/0x28
[ 3.365161] SMP: stopping secondary CPUs
[ 3.366180] Kernel Offset: disabled
[ 3.366784] CPU features: 0x002082
[ 3.367362] Memory Limit: none
[ 3.367897] Rebooting in 86400 seconds..
----

Syzkaller reproducer
----
mmap(&(0x7f0000000000/0xc72000)=nil, 0xc72000, 0x3, 0x32, 0xffffffffffffffff, 0x0)
r0 = userfaultfd(0x0)
ioctl$UFFDIO_API(r0, 0xc018aa3f, &(0x7f0000c08000-0x18)={0xaa, 0x2, 0x0})
setrlimit(0x7, &(0x7f0000000000)={0x0, 0x0})
ioctl$UFFDIO_REGISTER(r0, 0xc020aa00, &(0x7f0000010000)={{&(0x7f0000007000/0x3000)=nil, 0x3000}, 0x1, 0x0})
read(r0, &(0x7f0000015000-0x1000)="0000000000000000000000000000000000000000000000000000000000000000", 0x20)
clone(0x1000000, &(0x7f0000006000)="", &(0x7f0000012000)=0x0, &(0x7f0000014000-0x4)=0x0, &(0x7f0000011000-0x25)="")
----

C reproducer
----
#include <linux/userfaultfd.h>
#include <pthread.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <sys/resource.h>
#include <sys/syscall.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>

int userfaultfd(int flags)
{
return syscall(__NR_userfaultfd, flags);
}

// Only exists since v4.10, so not in most distro headers...
#ifndef UFFD_FEATURE_EVENT_FORK
#define UFFD_FEATURE_EVENT_FORK 2
#endif

// Arbitrary; needs to be some PAGE_SIZE multiple
#define REGION_SIZE (2 * 1024 * 1024)

int uffd_setup(void *base, size_t size)
{
int uffd;
struct uffdio_api ufa = {
.api = UFFD_API,
.features = UFFD_FEATURE_EVENT_FORK,
.ioctls = 0,
};
struct uffdio_register ufr = {
.range.start = (unsigned long)base,
.range.len = size,
.mode = UFFDIO_REGISTER_MODE_MISSING,
.ioctls = 0,
};

uffd = userfaultfd(0);
ioctl(uffd, UFFDIO_API, &ufa);
ioctl(uffd, UFFDIO_REGISTER, &ufr);

return uffd;
}

void *thr_uffd(void *unused)
{
void *base;
int uffd;
struct uffd_msg msg;
struct rlimit rlimit;

base = mmap(NULL, REGION_SIZE, PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);

uffd = uffd_setup(base, REGION_SIZE);

rlimit.rlim_cur = 0;
rlimit.rlim_max = 0;
setrlimit(RLIMIT_NOFILE, &rlimit);

read(uffd, &msg, sizeof(msg));

return NULL;
}

void *thr_clone(void *unused)
{
fork();
return NULL;
}

int main(int argc, char *argv[])
{
pthread_t p_uffd, p_clone;

pthread_create(&p_uffd, 0, thr_uffd, NULL);
usleep(1000);
pthread_create(&p_clone, 0, thr_clone, NULL);
usleep(1000);

return 0;
}
----

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