This is a note to let you know that I've just added the patch titled
binder: fix UAF of alloc->vma in race with munmap()
to my char-misc git tree which can be found at git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/char-misc.git in the char-misc-linus branch.
The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)
The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.
If you have any questions about this process, please let me know.
From d1d8875c8c13517f6fd1ff8d4d3e1ac366a17e07 Mon Sep 17 00:00:00 2001 From: Carlos Llamas cmllamas@google.com Date: Fri, 19 May 2023 19:59:49 +0000 Subject: binder: fix UAF of alloc->vma in race with munmap()
[ cmllamas: clean forward port from commit 015ac18be7de ("binder: fix UAF of alloc->vma in race with munmap()") in 5.10 stable. It is needed in mainline after the revert of commit a43cfc87caaf ("android: binder: stop saving a pointer to the VMA") as pointed out by Liam. The commit log and tags have been tweaked to reflect this. ]
In commit 720c24192404 ("ANDROID: binder: change down_write to down_read") binder assumed the mmap read lock is sufficient to protect alloc->vma inside binder_update_page_range(). This used to be accurate until commit dd2283f2605e ("mm: mmap: zap pages with read mmap_sem in munmap"), which now downgrades the mmap_lock after detaching the vma from the rbtree in munmap(). Then it proceeds to teardown and free the vma with only the read lock held.
This means that accesses to alloc->vma in binder_update_page_range() now will race with vm_area_free() in munmap() and can cause a UAF as shown in the following KASAN trace:
================================================================== BUG: KASAN: use-after-free in vm_insert_page+0x7c/0x1f0 Read of size 8 at addr ffff16204ad00600 by task server/558
CPU: 3 PID: 558 Comm: server Not tainted 5.10.150-00001-gdc8dcf942daa #1 Hardware name: linux,dummy-virt (DT) Call trace: dump_backtrace+0x0/0x2a0 show_stack+0x18/0x2c dump_stack+0xf8/0x164 print_address_description.constprop.0+0x9c/0x538 kasan_report+0x120/0x200 __asan_load8+0xa0/0xc4 vm_insert_page+0x7c/0x1f0 binder_update_page_range+0x278/0x50c binder_alloc_new_buf+0x3f0/0xba0 binder_transaction+0x64c/0x3040 binder_thread_write+0x924/0x2020 binder_ioctl+0x1610/0x2e5c __arm64_sys_ioctl+0xd4/0x120 el0_svc_common.constprop.0+0xac/0x270 do_el0_svc+0x38/0xa0 el0_svc+0x1c/0x2c el0_sync_handler+0xe8/0x114 el0_sync+0x180/0x1c0
Allocated by task 559: kasan_save_stack+0x38/0x6c __kasan_kmalloc.constprop.0+0xe4/0xf0 kasan_slab_alloc+0x18/0x2c kmem_cache_alloc+0x1b0/0x2d0 vm_area_alloc+0x28/0x94 mmap_region+0x378/0x920 do_mmap+0x3f0/0x600 vm_mmap_pgoff+0x150/0x17c ksys_mmap_pgoff+0x284/0x2dc __arm64_sys_mmap+0x84/0xa4 el0_svc_common.constprop.0+0xac/0x270 do_el0_svc+0x38/0xa0 el0_svc+0x1c/0x2c el0_sync_handler+0xe8/0x114 el0_sync+0x180/0x1c0
Freed by task 560: kasan_save_stack+0x38/0x6c kasan_set_track+0x28/0x40 kasan_set_free_info+0x24/0x4c __kasan_slab_free+0x100/0x164 kasan_slab_free+0x14/0x20 kmem_cache_free+0xc4/0x34c vm_area_free+0x1c/0x2c remove_vma+0x7c/0x94 __do_munmap+0x358/0x710 __vm_munmap+0xbc/0x130 __arm64_sys_munmap+0x4c/0x64 el0_svc_common.constprop.0+0xac/0x270 do_el0_svc+0x38/0xa0 el0_svc+0x1c/0x2c el0_sync_handler+0xe8/0x114 el0_sync+0x180/0x1c0
[...] ==================================================================
To prevent the race above, revert back to taking the mmap write lock inside binder_update_page_range(). One might expect an increase of mmap lock contention. However, binder already serializes these calls via top level alloc->mutex. Also, there was no performance impact shown when running the binder benchmark tests.
Fixes: c0fd2101781e ("Revert "android: binder: stop saving a pointer to the VMA"") Fixes: dd2283f2605e ("mm: mmap: zap pages with read mmap_sem in munmap") Reported-by: Jann Horn jannh@google.com Closes: https://lore.kernel.org/all/20230518144052.xkj6vmddccq4v66b@revolver Cc: stable@vger.kernel.org Cc: Minchan Kim minchan@kernel.org Cc: Yang Shi yang.shi@linux.alibaba.com Cc: Liam Howlett liam.howlett@oracle.com Signed-off-by: Carlos Llamas cmllamas@google.com Acked-by: Todd Kjos tkjos@google.com Link: https://lore.kernel.org/r/20230519195950.1775656-1-cmllamas@google.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/android/binder_alloc.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-)
diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c index e7c9d466f8e8..662a2a2e2e84 100644 --- a/drivers/android/binder_alloc.c +++ b/drivers/android/binder_alloc.c @@ -212,7 +212,7 @@ static int binder_update_page_range(struct binder_alloc *alloc, int allocate, mm = alloc->mm;
if (mm) { - mmap_read_lock(mm); + mmap_write_lock(mm); vma = alloc->vma; }
@@ -270,7 +270,7 @@ static int binder_update_page_range(struct binder_alloc *alloc, int allocate, trace_binder_alloc_page_end(alloc, index); } if (mm) { - mmap_read_unlock(mm); + mmap_write_unlock(mm); mmput(mm); } return 0; @@ -303,7 +303,7 @@ static int binder_update_page_range(struct binder_alloc *alloc, int allocate, } err_no_vma: if (mm) { - mmap_read_unlock(mm); + mmap_write_unlock(mm); mmput(mm); } return vma ? -ENOMEM : -ESRCH;