From: Eric Biggers <ebiggers@xxxxxxxxxx>
Commit 7c051267931a ("mm, fork: make dup_mmap wait for mmap_sem for
write killable") made it possible to kill a forking task while it is
waiting to acquire its ->mmap_sem for write, in dup_mmap().
However, it was overlooked that this introduced an new error path before
the new mm_struct's ->uprobes_state.xol_area has been set to NULL after
being copied from the old mm_struct by the memcpy in dup_mm(). For a
task that has previously hit a uprobe tracepoint, this resulted in the
'struct xol_area' being freed multiple times if the task was killed at
just the right time while forking.
Fix it by setting ->uprobes_state.xol_area to NULL in mm_init() rather
than in uprobe_dup_mmap().
With CONFIG_UPROBE_EVENTS=y, the bug can be reproduced by the same C
program given by commit 2b7e8665b4ff ("fork: fix incorrect fput of
->exe_file causing use-after-free"), provided that a uprobe tracepoint
has been set on the fork_thread() function. For example:
$ gcc reproducer.c -o reproducer -lpthread
$ nm reproducer | grep fork_thread
0000000000400719 t fork_thread
$ echo "p $PWD/reproducer:0x719" > /sys/kernel/debug/tracing/uprobe_events
$ echo 1 > /sys/kernel/debug/tracing/events/uprobes/enable
$ ./reproducer
Here is the use-after-free reported by KASAN:
BUG: KASAN: use-after-free in uprobe_clear_state+0x1c4/0x200
Read of size 8 at addr ffff8800320a8b88 by task reproducer/198
CPU: 1 PID: 198 Comm: reproducer Not tainted 4.13.0-rc7-00015-g36fde05f3fb5 #255
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-20170228_101828-anatol 04/01/2014
Call Trace:
dump_stack+0xdb/0x185
print_address_description+0x7e/0x290
kasan_report+0x23b/0x350
__asan_report_load8_noabort+0x19/0x20
uprobe_clear_state+0x1c4/0x200
mmput+0xd6/0x360
do_exit+0x740/0x1670
do_group_exit+0x13f/0x380
get_signal+0x597/0x17d0
do_signal+0x99/0x1df0
exit_to_usermode_loop+0x166/0x1e0
syscall_return_slowpath+0x258/0x2c0
entry_SYSCALL_64_fastpath+0xbc/0xbe
...
Allocated by task 199:
save_stack_trace+0x1b/0x20
kasan_kmalloc+0xfc/0x180
kmem_cache_alloc_trace+0xf3/0x330
__create_xol_area+0x10f/0x780
uprobe_notify_resume+0x1674/0x2210
exit_to_usermode_loop+0x150/0x1e0
prepare_exit_to_usermode+0x14b/0x180
retint_user+0x8/0x20
Freed by task 199:
save_stack_trace+0x1b/0x20
kasan_slab_free+0xa8/0x1a0
kfree+0xba/0x210
uprobe_clear_state+0x151/0x200
mmput+0xd6/0x360
copy_process.part.8+0x605f/0x65d0
_do_fork+0x1a5/0xbd0
SyS_clone+0x19/0x20
do_syscall_64+0x22f/0x660
return_from_SYSCALL_64+0x0/0x7a
Note: without KASAN, you may instead see a "Bad page state" message, or
simply a general protection fault.
Fixes: 7c051267931a ("mm, fork: make dup_mmap wait for mmap_sem for write killable")
Reported-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Konstantin Khlebnikov <koct9i@xxxxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> [v4.7+]
Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
---
kernel/events/uprobes.c | 2 --
kernel/fork.c | 8 ++++++++
2 files changed, 8 insertions(+), 2 deletions(-)
diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 0e137f98a50c..267f6ef91d97 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -1262,8 +1262,6 @@ void uprobe_end_dup_mmap(void)
void uprobe_dup_mmap(struct mm_struct *oldmm, struct mm_struct *newmm)
{
- newmm->uprobes_state.xol_area = NULL;
-
if (test_bit(MMF_HAS_UPROBES, &oldmm->flags)) {
set_bit(MMF_HAS_UPROBES, &newmm->flags);
/* unconditionally, dup_mmap() skips VM_DONTCOPY vmas */
diff --git a/kernel/fork.c b/kernel/fork.c
index cbbea277b3fb..b7e9e57b71ea 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -785,6 +785,13 @@ static void mm_init_owner(struct mm_struct *mm, struct task_struct *p)
#endif
}
+static void mm_init_uprobes_state(struct mm_struct *mm)
+{
+#ifdef CONFIG_UPROBES
+ mm->uprobes_state.xol_area = NULL;
+#endif
+}
+
static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
struct user_namespace *user_ns)
{
@@ -812,6 +819,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && !USE_SPLIT_PMD_PTLOCKS
mm->pmd_huge_pte = NULL;
#endif
+ mm_init_uprobes_state(mm);
if (current->mm) {
mm->flags = current->mm->flags & MMF_INIT_MASK;
--
2.14.1