uprobes: __replace_page() avoid BUG in munlock_vma_page()

commit c17c3dc9d08b9aad9a55a1e53f205187972f448e upstream.

syzbot crashed on the VM_BUG_ON_PAGE(PageTail) in munlock_vma_page(), when
called from uprobes __replace_page().  Which of many ways to fix it?
Settled on not calling when PageCompound (since Head and Tail are equals
in this context, PageCompound the usual check in uprobes.c, and the prior
use of FOLL_SPLIT_PMD will have cleared PageMlocked already).

Fixes: 5a52c9df62b4 ("uprobe: use FOLL_SPLIT_PMD instead of FOLL_SPLIT")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: <stable@vger.kernel.org>	[5.4+]
Link: http://lkml.kernel.org/r/alpine.LSU.2.11.2008161338360.20413@eggly.anvils
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index aa83538efc23824912cef6cfa86c46c04595656d..a793bd23fe56cf6ec7914df35f2e200dc56e4aaf 100644 (file)
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -211,7 +211,7 @@ static int __replace_page(struct vm_area_struct *vma, unsigned long addr,
                try_to_free_swap(old_page);
        page_vma_mapped_walk_done(&pvmw);
 
-       if (vma->vm_flags & VM_LOCKED)
+       if ((vma->vm_flags & VM_LOCKED) && !PageCompound(old_page))
                munlock_vma_page(old_page);
        put_page(old_page);