• Eric Biggers's avatar
    mm, uprobes: fix multiple free of ->uprobes_state.xol_area · 355627f5
    Eric Biggers authored
    Commit 7c051267 ("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 2b7e8665 ("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-g36fde05f #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.
    
    Link: http://lkml.kernel.org/r/20170830033303.17927-1-ebiggers3@gmail.com
    Fixes: 7c051267 ("mm, fork: make dup_mmap wait for mmap_sem for write killable")
    Signed-off-by: default avatarEric Biggers <ebiggers@google.com>
    Reported-by: default avatarOleg Nesterov <oleg@redhat.com>
    Acked-by: default avatarOleg Nesterov <oleg@redhat.com>
    Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
    Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
    Cc: Dmitry Vyukov <dvyukov@google.com>
    Cc: Ingo Molnar <mingo@kernel.org>
    Cc: Konstantin Khlebnikov <koct9i@gmail.com>
    Cc: Mark Rutland <mark.rutland@arm.com>
    Cc: Michal Hocko <mhocko@suse.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Vlastimil Babka <vbabka@suse.cz>
    Cc: <stable@vger.kernel.org>    [4.7+]
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    355627f5
fork.c 58.8 KB