uprobes: __replace_page() avoid BUG in munlock_vma_page()

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>

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 649fd53dc9ad047d24144e61275cfb9bd124e9d8..0e18aaf23a7b48c5d28739e78bb1b1d5b00f7978 100644 (file)
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -205,7 +205,7 @@ 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);