On Thu, 12 Dec 2013 14:07:57 -0800 Kees Cook <keescook@xxxxxxxxxxxx> wrote: > From: PaX Team <pageexec@xxxxxxxxxxx> > > http://lkml.org/lkml/2013/9/17/30 > > SyS_remap_file_pages() calls mmap_region(), which calls remove_vma_list(), > which calls remove_vma(), which frees the vma. Later (after out label) > SyS_remap_file_pages() accesses the freed vma in vm_flags = vma->vm_flags. > index 5bff08147768..afad07b85ef2 100644 > --- a/mm/fremap.c > +++ b/mm/fremap.c > @@ -218,6 +218,8 @@ get_write_lock: > BUG_ON(addr != start); > err = 0; > } > + vm_flags = vma->vm_flags; > + vma = NULL; > goto out; If the vma has been freed by the time the code jumps to the out label (because it was freed by a function called from mmap_region), surely it will also already have been freed by the time this patch dereferences it? Also, setting vma = NULL to avoid the if (vma) branch at the out: label is unnecessarily obfuscated. Lets make things clear by documenting what is going on, and having a label after that dereference. Maybe something like this patch: ---8<--- Subject: [PATCH] mm: fix use-after-free in sys_remap_file_pages remap_file_pages calls mmap_region, which may merge the VMA with other existing VMAs, and free "vma". This can lead to a use-after-free bug. Avoid the bug by remembering vm_flags before calling mmap_region, and not trying to dereference vma later. Signed-off-by: Rik van Riel <riel@xxxxxxxxxx> Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx> Cc: stable@xxxxxxxxxxxxxxx --- mm/fremap.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/mm/fremap.c b/mm/fremap.c index 87da359..d28ede5 100644 --- a/mm/fremap.c +++ b/mm/fremap.c @@ -203,6 +203,7 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size, if (mapping_cap_account_dirty(mapping)) { unsigned long addr; struct file *file = get_file(vma->vm_file); + vm_flags = vma->vm_flags; addr = mmap_region(file, start, size, vma->vm_flags, pgoff); @@ -213,7 +214,8 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size, BUG_ON(addr != start); err = 0; } - goto out; + /* mmap_region may have freed vma */ + goto out_freed; } mutex_lock(&mapping->i_mmap_mutex); flush_dcache_mmap_lock(mapping); @@ -248,6 +250,7 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size, out: if (vma) vm_flags = vma->vm_flags; +out_freed: if (likely(!has_write_lock)) up_read(&mm->mmap_sem); else -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>