path: root/kernel/fork.c
diff options
authorEric Biggers <ebiggers@google.com>2017-08-25 15:55:43 -0700
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2017-08-30 10:21:47 +0200
commitb65b6ac52e0f8694aa3a4402d5f766b2bb9e94ef (patch)
treef0617b9a1d74e64cffc13b564c60bd15f2fe87a4 /kernel/fork.c
parent0f49b0519f1190858f36e0bdafd4a139c9c2e34a (diff)
fork: fix incorrect fput of ->exe_file causing use-after-free
commit 2b7e8665b4ff51c034c55df3cff76518d1a9ee3a upstream. 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 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@gmail.com Fixes: 7c051267931a ("mm, fork: make dup_mmap wait for mmap_sem for write killable") Signed-off-by: Eric Biggers <ebiggers@google.com> Tested-by: Mark Rutland <mark.rutland@arm.com> Acked-by: Michal Hocko <mhocko@suse.com> Cc: Dmitry Vyukov <dvyukov@google.com> Cc: Ingo Molnar <mingo@kernel.org> Cc: Konstantin Khlebnikov <koct9i@gmail.com> Cc: Oleg Nesterov <oleg@redhat.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Vlastimil Babka <vbabka@suse.cz> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'kernel/fork.c')
1 files changed, 1 insertions, 0 deletions
diff --git a/kernel/fork.c b/kernel/fork.c
index 59faac4de181..50bf262cc427 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -766,6 +766,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
mm_init_owner(mm, p);
+ RCU_INIT_POINTER(mm->exe_file, NULL);