On Wed, 09 Jun 2021 11:51:34 +0100,
Steven Price [off-list ref] wrote:

On 09/06/2021 11:30, Marc Zyngier wrote:

quoted

On Mon, 07 Jun 2021 12:08:09 +0100,
Steven Price [off-list ref] wrote:

quoted

mte_sync_tags() used test_and_set_bit() to set the PG_mte_tagged flag
before restoring/zeroing the MTE tags. However if another thread were to
race and attempt to sync the tags on the same page before the first
thread had completed restoring/zeroing then it would see the flag is
already set and continue without waiting. This would potentially expose
the previous contents of the tags to user space, and cause any updates
that user space makes before the restoring/zeroing has completed to
potentially be lost.

Since this code is run from atomic contexts we can't just lock the page
during the process. Instead implement a new (global) spinlock to protect
the mte_sync_page_tags() function.

Fixes: 34bfeea4a9e9 ("arm64: mte: Clear the tags when a page is mapped in user-space with PROT_MTE")
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
---
 arch/arm64/kernel/mte.c | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index 125a10e413e9..a3583a7fd400 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c

@@ -25,6 +25,7 @@
 u64 gcr_kernel_excl __ro_after_init;
 
 static bool report_fault_once = true;
+static DEFINE_SPINLOCK(tag_sync_lock);
 
 #ifdef CONFIG_KASAN_HW_TAGS
 /* Whether the MTE asynchronous mode is enabled. */

@@ -34,13 +35,22 @@ EXPORT_SYMBOL_GPL(mte_async_mode);
 
 static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
 {
+	unsigned long flags;
 	pte_t old_pte = READ_ONCE(*ptep);
 
+	spin_lock_irqsave(&tag_sync_lock, flags);

having though a bit more about this after an offline discussion with
Catalin: why can't this lock be made per mm? We can't really share
tags across processes anyway, so this is limited to threads from the
same process.

Currently there's nothing stopping processes sharing tags (mmap(...,
PROT_MTE, MAP_SHARED)) - I agree making use of this is tricky and it
would have been nice if this had just been prevented from the
beginning.

I don't think it should be prevented. I think it should be made clear
that it is unreliable and that it will result in tag corruption.

Given the above, clearly the lock can't be per mm and robust.

I don't think we need to make it robust. The architecture actively
prevents sharing if the tags are also shared, just like we can't
really expect the VMM to share tags with the guest.

quoted

I'd also like it to be documented that page sharing can only reliably
work with tagging if only one of the mappings is using tags.

I'm not entirely clear whether you mean "can only reliably work" to be
"is practically impossible to coordinate tag values", or whether you are
proposing to (purposefully) introduce the race with a per-mm lock? (and
document it).

The latter. You can obviously communicate your tags to another task,
but this should come with attached restrictions (mlock?).

I guess we could have a per-mm lock and handle the race if user space
screws up with the outcome being lost tags (double clear).

But it feels to me like it could come back to bite in the future since
VM_SHARED|VM_MTE will almost always work and I fear someone will start
using it since it's permitted by the kernel.

I'm really worried that performance is going to suck even on a small
system, and this global lock will be heavily contended, even without
considering KVM.

Catalin, any though?

	M.

-- 
Without deviation from the norm, progress is not possible.

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