From c365ff7815401e376fced9c42716488d74dc84d5 Mon Sep 17 00:00:00 2001 From: Will Deacon Date: Wed, 25 Mar 2020 11:13:46 +0000 Subject: [PATCH] mm/mremap: Add comment explaining the untagging behaviour of mremap() 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 Acked-by: Linus Torvalds Reviewed-by: Catalin Marinas Signed-off-by: Will Deacon Signed-off-by: Catalin Marinas Signed-off-by: Greg Kroah-Hartman --- mm/mremap.c | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/mm/mremap.c b/mm/mremap.c index 1d98281f..245bf9c 100644 --- a/mm/mremap.c +++ b/mm/mremap.c @@ -606,6 +606,16 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len, 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)) -- 2.7.4