Re: [PATCH] arm64/pageattr: Propagate return value from __change_memory_common
From: Dev Jain
Date: Mon Nov 10 2025 - 23:55:44 EST
On 11/11/25 10:14 am, Yang Shi wrote:
On 11/10/25 8:37 PM, Dev Jain wrote:
On 11/11/25 9:47 am, Yang Shi wrote:
On 11/10/25 7:39 PM, Dev Jain wrote:
On 05/11/25 9:27 am, Dev Jain wrote:
On 04/11/25 6:26 pm, Will Deacon wrote:
On Tue, Nov 04, 2025 at 09:06:12AM +0530, Dev Jain wrote:
On 04/11/25 12:15 am, Yang Shi wrote:
On 11/3/25 7:16 AM, Will Deacon wrote:
On Mon, Nov 03, 2025 at 11:43:06AM +0530, Dev Jain wrote:
Post a166563e7ec3 ("arm64: mm: support large block mapping when
rodata=full"),
__change_memory_common has a real chance of failing due to split
failure.
Before that commit, this line was introduced in c55191e96caa,
still having
a chance of failing if it needs to allocate pagetable memory in
apply_to_page_range, although that has never been observed to
be true.
In general, we should always propagate the return value to
the caller.
Cc: stable@xxxxxxxxxxxxxxx
Fixes: c55191e96caa ("arm64: mm: apply r/o permissions of VM
areas to its linear alias as well")
Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
---
Based on Linux 6.18-rc4.
arch/arm64/mm/pageattr.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
index 5135f2d66958..b4ea86cd3a71 100644
--- a/arch/arm64/mm/pageattr.c
+++ b/arch/arm64/mm/pageattr.c
@@ -148,6 +148,7 @@ static int change_memory_common(unsigned
long addr, int numpages,
unsigned long size = PAGE_SIZE * numpages;
unsigned long end = start + size;
struct vm_struct *area;
+ int ret;
int i;
if (!PAGE_ALIGNED(addr)) {
@@ -185,8 +186,10 @@ static int change_memory_common(unsigned
long addr, int numpages,
if (rodata_full && (pgprot_val(set_mask) == PTE_RDONLY ||
pgprot_val(clear_mask) == PTE_RDONLY)) {
for (i = 0; i < area->nr_pages; i++) {
- __change_memory_common((u64)page_address(area->pages[i]),
+ ret =
__change_memory_common((u64)page_address(area->pages[i]),
PAGE_SIZE, set_mask, clear_mask);
+ if (ret)
+ return ret;
Hmm, this means we can return failure half-way through the
operation. Is
that something callers are expecting to handle? If so, how can
they tell
how far we got?
IIUC the callers don't have to know whether it is half-way or not
because the callers will change the permission back (e.g. to
RW) for the
whole range when freeing memory.
Yes, it is the caller's responsibility to set
VM_FLUSH_RESET_PERMS flag.
Upon vfree(), it will change the direct map permissions back to RW.
Ok, but vfree() ends up using update_range_prot() to do that and
if we
need to worry about that failing (as per your commit message), then
we're in trouble because the calls to set_area_direct_map() are
unchecked.
In other words, this patch is either not necessary or it is
incomplete.
Here is the relevant email, in the discussion between Ryan and Yang:
https://lore.kernel.org/all/fe52a1d8-5211-4962-afc8-c3f9caf64119@xxxxxxxxxxxxxxxxxxxxxx/
We had concluded that all callers of set_memory_ro() or
set_memory_rox() (which require the
linear map perm change back to default, upon vfree() ) will call
it for the entire region (vm_struct).
So, when we do the set_direct_map_invalid_noflush, it is
guaranteed that the region has already
been split. So this call cannot fail.
https://lore.kernel.org/all/f8898c87-8f49-4ef2-86ae-b60bcf67658c@xxxxxxxxxxxxxxxxxxxxxx/
This email notes that there is some code doing set_memory_rw() and
unnecessarily setting the VM_FLUSH_RESET_PERMS
flag, but in that case we don't care about the
set_direct_map_invalid_noflush call failing because the protections
are already RW.
Although we had also observed that all of this is fragile and
depends on the caller doing the
correct thing. The real solution should be somehow getting rid of
the BBM style invalidation.
Ryan had proposed some methods in that email thread.
One solution which I had thought of, is that, observe that we are
doing an overkill by
setting the linear map to invalid and then default, for the
*entire* region. What we
can do is iterate over the linear map alias of the vm_struct *area
and only change permission
back to RW for the pages which are *not* RW. And, those relevant
mappings are guaranteed to
be split because they were changed from RW to not RW.
@Yang and Ryan,
I saw Yang's patch here:
https://lore.kernel.org/all/20251023204428.477531-1-yang@xxxxxxxxxxxxxxxxxxxxxx/
and realized that currently we are splitting away the linear map
alias of the *entire* region.
Shouldn't this then imply that set_direct_map_invalid_noflush will
never fail, since even
a set_memory_rox() call on a single page will split the linear map
for the entire region,
and thus there is no fragility here which we were discussing about?
I may be forgetting
something, this linear map stuff is confusing enough already.
It still may fail due to page table allocation failure when doing
split. But it is still fine. We may run into 3 cases:
1. set_memory_rox succeed to split the whole range, then
set_direct_map_invalid_noflush() will succeed too
2. set_memory_rox fails to split, for example, just change partial
range permission due to page table allocation failure, then
set_direct_map_invalid_noflush() may
a. successfully change the permission back to default till where
set_memory_rox fails at since that range has been successfully
split. It is ok since the remaining range is actually not changed to
ro by set_memory_rox at all
b. successfully change the permission back to default for the
whole range (for example, memory pressure is mitigated when
set_direct_map_invalid_noflush() is called). It is definitely fine
as well
Correct, what I mean to imply here is that, your patch will break
this? If set_memory_* is applied on x till y, your patch changes the
linear map alias
only from x till y - set_direct_map_invalid_noflush instead operates
on 0 till size - 1, where 0 <=x <=y <= size - 1. So, it may encounter
a -ENOMEM
on [0, x) range while invalidating, and that is *not* okay because we
must reset back [0, x) to default?
I see your point now. But I think the callers need to guarantee they
call set_memory_rox and set_direct_map_invalid_noflush on the same
range, right? Currently kernel just calls them on the whole area.
Nope. The fact that the kernel changes protections, and undoes the
changed protections, on the *entire* alias of the vm_struct region,
protects us from the fragility we were talking about earlier.
Suppose you have a range from 0 till size - 1, and you call set_memory_*
on a random point (page) p. The argument we discussed above is
independent of p, which lets us drop our
previous erroneous conclusion that all of this works because no caller
does a partial set_memory_*.
I would like to send a patch clearly documenting this behaviour,
assuming no one else finds a hole in this reasoning.
Thanks,
Yang
Hopefully I don't miss anything.
Thanks,
Yang
Will