Subject: Re: [PATCH net] page_pool: Fix PP_MAGIC_MASK to avoid crashing on some 32-bit arches

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Tue, 30 Sep 2025 09:45:42 +0200

Mina Almasry <[email-masked]> writes:

> On Fri, Sep 26, 2025 at 4:40 AM Toke Høiland-Jørgensen <[email-masked]> wrote:
>>
>> Helge reported that the introduction of PP_MAGIC_MASK let to crashes on
>> boot on his 32-bit parisc machine. The cause of this is the mask is set
>> too wide, so the page_pool_page_is_pp() incurs false positives which
>> crashes the machine.
>>
>> Just disabling the check in page_pool_is_pp() will lead to the page_pool
>> code itself malfunctioning; so instead of doing this, this patch changes
>> the define for PP_DMA_INDEX_BITS to avoid mistaking arbitrary kernel
>> pointers for page_pool-tagged pages.
>>
>> The fix relies on the kernel pointers that alias with the pp_magic field
>> always being above PAGE_OFFSET. With this assumption, we can use the
>> lowest bit of the value of PAGE_OFFSET as the upper bound of the
>> PP_DMA_INDEX_MASK, which should avoid the false positives.
>>
>> Because we cannot rely on PAGE_OFFSET always being a compile-time
>> constant, nor on it always being >0, we fall back to disabling the
>> dma_index storage when there are no bits available. This leaves us in
>> the situation we were in before the patch in the Fixes tag, but only on
>> a subset of architecture configurations. This seems to be the best we
>> can do until the transition to page types in complete for page_pool
>> pages.
>>
>> Link: https://lore.kernel.org/all/aMNJMFa5fDalFmtn@p100/
>> Fixes: ee62ce7a1d90 ("page_pool: Track DMA-mapped pages and unmap them when destroying the pool")
>> Signed-off-by: Toke Høiland-Jørgensen <[email-masked]>
>> ---
>> Sorry for the delay on getting this out. I have only compile-tested it,
>> since I don't have any hardware that triggers the original bug. Helge, I'm
>> hoping you can take it for a spin?
>>
>>  include/linux/mm.h   | 18 +++++------
>>  net/core/page_pool.c | 76 ++++++++++++++++++++++++++++++--------------
>>  2 files changed, 62 insertions(+), 32 deletions(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 1ae97a0b8ec7..28541cb40f69 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -4159,14 +4159,13 @@ int arch_lock_shadow_stack_status(struct task_struct *t, unsigned long status);
>>   * since this value becomes part of PP_SIGNATURE; meaning we can just use the
>>   * space between the PP_SIGNATURE value (without POISON_POINTER_DELTA), and the
>>   * lowest bits of POISON_POINTER_DELTA. On arches where POISON_POINTER_DELTA is
>> - * 0, we make sure that we leave the two topmost bits empty, as that guarantees
>> - * we won't mistake a valid kernel pointer for a value we set, regardless of the
>> - * VMSPLIT setting.
>> + * 0, we use the lowest bit of PAGE_OFFSET as the boundary if that value is
>> + * known at compile-time.
>>   *
>> - * Altogether, this means that the number of bits available is constrained by
>> - * the size of an unsigned long (at the upper end, subtracting two bits per the
>> - * above), and the definition of PP_SIGNATURE (with or without
>> - * POISON_POINTER_DELTA).
>> + * If the value of PAGE_OFFSET is not known at compile time, or if it is too
>> + * small to leave some bits available above PP_SIGNATURE, we define the number
>> + * of bits to be 0, which turns off the DMA index tracking altogether (see
>> + * page_pool_register_dma_index()).
>>   */
>>  #define PP_DMA_INDEX_SHIFT (1 + __fls(PP_SIGNATURE - POISON_POINTER_DELTA))
>>  #if POISON_POINTER_DELTA > 0
>> @@ -4175,8 +4174,9 @@ int arch_lock_shadow_stack_status(struct task_struct *t, unsigned long status);
>>   */
>>  #define PP_DMA_INDEX_BITS MIN(32, __ffs(POISON_POINTER_DELTA) - PP_DMA_INDEX_SHIFT)
>>  #else
>> -/* Always leave out the topmost two; see above. */
>> -#define PP_DMA_INDEX_BITS MIN(32, BITS_PER_LONG - PP_DMA_INDEX_SHIFT - 2)
>> +/* Constrain to the lowest bit of PAGE_OFFSET if known; see above. */
>> +#define PP_DMA_INDEX_BITS ((__builtin_constant_p(PAGE_OFFSET) && PAGE_OFFSET > PP_SIGNATURE) ? \
>> +                             MIN(32, __ffs(PAGE_OFFSET) - PP_DMA_INDEX_SHIFT) : 0)
>
> Do you have to watch out for an underflow of __ffs(PAGE_OFFSET) -
> PP_DMA_INDEX_SHIFT (at which point we'll presumably use 32 here
> instead of the expected 0)? Or is that guaranteed to be positive for
> some reason I'm not immediately grasping.

That's what the 'PAGE_OFFSET > PP_SIGNATURE' in the ternary operator is
for. I'm assuming that PAGE_OFFSET is always a "round" number (e.g.,
0xc0000000), in which case that condition should be sufficient, no?

-Toke