Re: [PATCH hotfix 6.12 v2 4/8] mm: resolve faulty mmap_region() error path behaviour

On Tue, Oct 29, 2024 at 03:04:41PM +0000, Catalin Marinas wrote:

Hi Lorenzo,

Thanks for trying to fix this.

On Mon, Oct 28, 2024 at 10:14:50PM +0000, Lorenzo Stoakes wrote:

quoted

From 247003cd2a4b5f4fc2dac97f5ef7e473a47f4324 Mon Sep 17 00:00:00 2001
From: Lorenzo Stoakes <redacted>
Date: Mon, 28 Oct 2024 22:05:44 +0000
Subject: [PATCH] mm: perform MTE check within arm64 hook entirely

It doesn't make sense to have shmem explicitly check this one arch-specific
case, it is arch-specific, so the arch should handle it. We know shmem is a
case in which we want to permit MTE, so simply check for this directly.

This also fixes the issue with checking arch_validate_flags() early, which
would otherwise break mmap_region().

In order to implement this we must pass a file pointer, and additionally
update the sparc code to accept this parameter too.

We'd ideally like to have eliminated the arch_calc_vm_flag_bits() case, but
we risk inadvertently changing behaviour as we do not have mmap() flags
available at the point of the arch_validate_flags() check and a MAP_ANON |
MAP_HUGETLB case would be accepted for MTE currently, but a MAP_SHARED |
MAP_HUGETLB would not.

This is likely an oversight but we want to try to keep behaviour identical
to before in this patch.

MAP_HUGETLB support for MTE is only in -next currently, so there
wouldn't be any ABI change if we also allowed MAP_SHARED | MAP_HUGETLB.
In 6.12, MAP_HUGETLB is not allowed to have PROT_MTE.

quoted

So continue to check VM_MTE_ALLOWED which arch_calc_vm_flag_bits() sets if
MAP_ANON.

[...]

quoted

diff --git a/mm/shmem.c b/mm/shmem.c
index 4ba1d00fabda..e87f5d6799a7 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c

@@ -2733,9 +2733,6 @@ static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
 	if (ret)
 		return ret;

-	/* arm64 - allow memory tagging on RAM-based files */
-	vm_flags_set(vma, VM_MTE_ALLOWED);

This breaks arm64 KVM if the VMM uses shared mappings for the memory
slots (which is possible). We have kvm_vma_mte_allowed() that checks for
the VM_MTE_ALLOWED flag as the VMM may not use PROT_MTE/VM_MTE directly.

Ugh yup missed that thanks.

I need to read this thread properly but why not pass the file argument
to arch_calc_vm_flag_bits() and set VM_MTE_ALLOWED in there?

Can't really do that as it is entangled in a bunch of other stuff,
e.g. calc_vm_prot_bits() would have to pass file and that's used in a bunch
of places including arch code and... etc. etc.

And definitely no good for a hotfix that has to be backported.

I suggest instead we instead don't drop the yucky shmem thing, which will
set VM_MTE_ALLOWED for shmem, with arch_calc_vm_flag_bits() still setting
it for MAP_ANON, but the other changes will mean the arch_validate_flags()
will be fixed too.

So this just means not dropping the mm/shmem.c bit basically and everything
should 'just work'?

--
Catalin

But we definitely need to find a better way post-hotfix to sort all this
out I'm sure you agree :)