Re: [PATCH v3 1/7] arm64: mte: Fix/clarify the PG_mte_tagged semantics

On Wed, Aug 10, 2022 at 12:30:27PM -0700, Peter Collingbourne wrote:

From: Catalin Marinas <catalin.marinas@arm.com>

Currently the PG_mte_tagged page flag mostly means the page contains
valid tags and it should be set after the tags have been cleared or
restored. However, in mte_sync_tags() it is set before setting the tags
to avoid, in theory, a race with concurrent mprotect(PROT_MTE) for
shared pages. However, a concurrent mprotect(PROT_MTE) with a copy on
write in another thread can cause the new page to have stale tags.
Similarly, tag reading via ptrace() can read stale tags of the
PG_mte_tagged flag is set before actually clearing/restoring the tags.

Fix the PG_mte_tagged semantics so that it is only set after the tags
have been cleared or restored. This is safe for swap restoring into a
MAP_SHARED or CoW page since the core code takes the page lock. Add two
functions to test and set the PG_mte_tagged flag with acquire and
release semantics. The downside is that concurrent mprotect(PROT_MTE) on
a MAP_SHARED page may cause tag loss. This is already the case for KVM
guests if a VMM changes the page protection while the guest triggers a
user_mem_abort().

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Steven Price <steven.price@arm.com>
Cc: Peter Collingbourne <redacted>
---
v3:
- fix build with CONFIG_ARM64_MTE disabled

When you post someone else's patches (thanks for updating them BTW),
please add your Signed-off-by line. You should also add a note in the
SoB block about the changes you made, so something like:

[pcc@google.com: fix build with CONFIG_ARM64_MTE disabled]
Singed-off-by: your name/address

-- 
Catalin

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