From: Eric Biggers <ebiggers@xxxxxxxxxx>
Subject: fork: fix incorrect fput of ->exe_file causing use-after-free
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 a reference is
taken on the mm_struct's ->exe_file. Since the ->exe_file of the new
mm_struct was already set to the old ->exe_file by the memcpy() in
dup_mm(), it was possible for the mmput() in the error path of dup_mm() to
drop a reference to ->exe_file which was never taken. This caused the
struct file to later be freed prematurely.
Fix it by updating mm_init() to NULL out the ->exe_file, in the same place
it clears other things like the list of mmaps.
This bug was found by syzkaller. It can be reproduced using the
following C program:
#define _GNU_SOURCE
#include <pthread.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/syscall.h>
#include <sys/wait.h>
#include <unistd.h>
static void *mmap_thread(void *_arg)
{
for (;;) {
mmap(NULL, 0x1000000, PROT_READ,
MAP_POPULATE|MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
}
}
static void *fork_thread(void *_arg)
{
usleep(rand() % 10000);
fork();
}
int main(void)
{
fork();
fork();
fork();
for (;;) {
if (fork() == 0) {
pthread_t t;
pthread_create(&t, NULL, mmap_thread, NULL);
pthread_create(&t, NULL, fork_thread, NULL);
usleep(rand() % 10000);
syscall(__NR_exit_group, 0);
}
wait(NULL);
}
}
No special kernel config options are needed. It usually causes a NULL
pointer dereference in __remove_shared_vm_struct() during exit, or in
dup_mmap() (which is usually inlined into copy_process()) during fork.
Both are due to a vm_area_struct's ->vm_file being used after it's already
been freed.
Google Bug Id: 64772007
Link: http://lkml.kernel.org/r/20170823211408.31198-1-ebiggers3@xxxxxxxxx
Fixes: 7c051267931a ("mm, fork: make dup_mmap wait for mmap_sem for write killable")
Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
Tested-by: Mark Rutland <mark.rutland@xxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Konstantin Khlebnikov <koct9i@xxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> [v4.7+]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
kernel/fork.c | 1 +
1 file changed, 1 insertion(+)
diff -puN kernel/fork.c~fork-fix-incorrect-fput-of-exe_file-causing-use-after-free kernel/fork.c
--- a/kernel/fork.c~fork-fix-incorrect-fput-of-exe_file-causing-use-after-free
+++ a/kernel/fork.c
@@ -806,6 +806,7 @@ static struct mm_struct *mm_init(struct
mm_init_cpumask(mm);
mm_init_aio(mm);
mm_init_owner(mm, p);
+ RCU_INIT_POINTER(mm->exe_file, NULL);
mmu_notifier_mm_init(mm);
init_tlb_flush_pending(mm);
#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && !USE_SPLIT_PMD_PTLOCKS
_
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