From: Will Deacon will@kernel.org
commit b2a84de2a2deb76a6a51609845341f508c518c03 upstream.
Commit dcde237319e6 ("mm: Avoid creating virtual address aliases in brk()/mmap()/mremap()") changed mremap() so that only the 'old' address is untagged, leaving the 'new' address in the form it was passed from userspace. This prevents the unexpected creation of aliasing virtual mappings in userspace, but looks a bit odd when you read the code.
Add a comment justifying the untagging behaviour in mremap().
Reported-by: Linus Torvalds torvalds@linux-foundation.org Acked-by: Linus Torvalds torvalds@linux-foundation.org Reviewed-by: Catalin Marinas catalin.marinas@arm.com Signed-off-by: Will Deacon will@kernel.org Signed-off-by: Catalin Marinas catalin.marinas@arm.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- mm/mremap.c | 10 ++++++++++ 1 file changed, 10 insertions(+)
--- a/mm/mremap.c +++ b/mm/mremap.c @@ -606,6 +606,16 @@ SYSCALL_DEFINE5(mremap, unsigned long, a LIST_HEAD(uf_unmap_early); LIST_HEAD(uf_unmap);
+ /* + * There is a deliberate asymmetry here: we strip the pointer tag + * from the old address but leave the new address alone. This is + * for consistency with mmap(), where we prevent the creation of + * aliasing mappings in userspace by leaving the tag bits of the + * mapping address intact. A non-zero tag will cause the subsequent + * range checks to reject the address as invalid. + * + * See Documentation/arm64/tagged-address-abi.rst for more information. + */ addr = untagged_addr(addr);
if (flags & ~(MREMAP_FIXED | MREMAP_MAYMOVE))