Re: [PATCH 1/5] io_uring: Adjust mapping wrt architecture aliasing requirements

Helge Deller
2023-07-12T19:06:12Z

On 7/12/23 19:28, matoro wrote:
> On 2023-07-12 12:24, Helge Deller wrote:
>> Hi Matoro,
>>
>> * matoro <matoro_mailinglist_kernel@matoro.tk>:
>>> On 2023-03-14 13:16, Jens Axboe wrote:
>>> > From: Helge Deller <deller@gmx.de>
>>> >
>>> > Some architectures have memory cache aliasing requirements (e.g. parisc)
>>> > if memory is shared between userspace and kernel. This patch fixes the
>>> > kernel to return an aliased address when asked by userspace via mmap().
>>> >
>>> > Signed-off-by: Helge Deller <deller@gmx.de>
>>> > Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>> > ---
>>> >  io_uring/io_uring.c | 51 +++++++++++++++++++++++++++++++++++++++++++++
>>> >  1 file changed, 51 insertions(+)
>>> >
>>> > diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
>>> > index 722624b6d0dc..3adecebbac71 100644
>>> > --- a/io_uring/io_uring.c
>>> > +++ b/io_uring/io_uring.c
>>> > @@ -72,6 +72,7 @@
>>> >  #include <linux/io_uring.h>
>>> >  #include <linux/audit.h>
>>> >  #include <linux/security.h>
>>> > +#include <asm/shmparam.h>
>>> >
>>> >  #define CREATE_TRACE_POINTS
>>> >  #include <trace/events/io_uring.h>
>>> > @@ -3317,6 +3318,54 @@ static __cold int io_uring_mmap(struct file
>>> > *file, struct vm_area_struct *vma)
>>> >      return remap_pfn_range(vma, vma->vm_start, pfn, sz,
>>> > vma->vm_page_prot);
>>> >  }
>>> >
>>> > +static unsigned long io_uring_mmu_get_unmapped_area(struct file *filp,
>>> > +            unsigned long addr, unsigned long len,
>>> > +            unsigned long pgoff, unsigned long flags)
>>> > +{
>>> > +    const unsigned long mmap_end = arch_get_mmap_end(addr, len, flags);
>>> > +    struct vm_unmapped_area_info info;
>>> > +    void *ptr;
>>> > +
>>> > +    /*
>>> > +     * Do not allow to map to user-provided address to avoid breaking the
>>> > +     * aliasing rules. Userspace is not able to guess the offset address
>>> > of
>>> > +     * kernel kmalloc()ed memory area.
>>> > +     */
>>> > +    if (addr)
>>> > +        return -EINVAL;
>>> > +
>>> > +    ptr = io_uring_validate_mmap_request(filp, pgoff, len);
>>> > +    if (IS_ERR(ptr))
>>> > +        return -ENOMEM;
>>> > +
>>> > +    info.flags = VM_UNMAPPED_AREA_TOPDOWN;
>>> > +    info.length = len;
>>> > +    info.low_limit = max(PAGE_SIZE, mmap_min_addr);
>>> > +    info.high_limit = arch_get_mmap_base(addr, current->mm->mmap_base);
>>> > +#ifdef SHM_COLOUR
>>> > +    info.align_mask = PAGE_MASK & (SHM_COLOUR - 1UL);
>>> > +#else
>>> > +    info.align_mask = PAGE_MASK & (SHMLBA - 1UL);
>>> > +#endif
>>> > +    info.align_offset = (unsigned long) ptr;
>>> > +
>>> > +    /*
>>> > +     * A failed mmap() very likely causes application failure,
>>> > +     * so fall back to the bottom-up function here. This scenario
>>> > +     * can happen with large stack limits and large mmap()
>>> > +     * allocations.
>>> > +     */
>>> > +    addr = vm_unmapped_area(&info);
>>> > +    if (offset_in_page(addr)) {
>>> > +        info.flags = 0;
>>> > +        info.low_limit = TASK_UNMAPPED_BASE;
>>> > +        info.high_limit = mmap_end;
>>> > +        addr = vm_unmapped_area(&info);
>>> > +    }
>>> > +
>>> > +    return addr;
>>> > +}
>>> > +
>>> >  #else /* !CONFIG_MMU */
>>> >
>>> >  static int io_uring_mmap(struct file *file, struct vm_area_struct *vma)
>>> > @@ -3529,6 +3578,8 @@ static const struct file_operations io_uring_fops
>>> > = {
>>> >  #ifndef CONFIG_MMU
>>> >      .get_unmapped_area = io_uring_nommu_get_unmapped_area,
>>> >      .mmap_capabilities = io_uring_nommu_mmap_capabilities,
>>> > +#else
>>> > +    .get_unmapped_area = io_uring_mmu_get_unmapped_area,
>>> >  #endif
>>> >      .poll        = io_uring_poll,
>>> >  #ifdef CONFIG_PROC_FS
>>>
>>> Hi Jens, Helge - I've bisected a regression with io_uring on ia64 to this
>>> patch in 6.4.  Unfortunately this breaks userspace programs using io_uring,
>>> the easiest one to test is cmake with an io_uring enabled libuv (i.e., libuv
>>> >= 1.45.0) which will hang.
>>>
>>> I am aware that ia64 is in a vulnerable place right now which I why I am
>>> keeping this spread limited.  Since this clearly involves
>>> architecture-specific changes for parisc,
>>
>> it isn't so much architecture-specific... (just one ifdef)
>>
>>> is there any chance of looking at
>>> what is required to do the same for ia64?  I looked at
>>> 0ef36bd2b37815719e31a72d2beecc28ca8ecd26 ("parisc: change value of SHMLBA
>>> from 0x00400000 to PAGE_SIZE") and tried to replicate the SHMLBA ->
>>> SHM_COLOUR change, but it made no difference.
>>>
>>> If hardware is necessary for testing, I can provide it, including remote BMC
>>> access for restarts/kernel debugging.  Any takers?
>>
>> I won't have time to test myself, but maybe you could test?
>>
>> Basically we should try to find out why io_uring_mmu_get_unmapped_area()
>> doesn't return valid addresses, while arch_get_unmapped_area()
>> [in arch/ia64/kernel/sys_ia64.c] does.
>>
>> You could apply this patch first:
>> It introduces a memory leak (as it requests memory twice), but maybe we
>> get an idea?
>> The ia64 arch_get_unmapped_area() searches for memory from bottom
>> (flags=0), while io_uring function tries top-down first. Maybe that's
>> the problem. And I don't understand the offset_in_page() check right
>> now.
>>
>> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
>> index 3bca7a79efda..93b1964d2bbb 100644
>> --- a/io_uring/io_uring.c
>> +++ b/io_uring/io_uring.c
>> @@ -3431,13 +3431,17 @@ static unsigned long io_uring_mmu_get_unmapped_area(struct file *filp,
>>       * can happen with large stack limits and large mmap()
>>       * allocations.
>>       */
>> +/* compare to arch_get_unmapped_area() in arch/ia64/kernel/sys_ia64.c */
>>      addr = vm_unmapped_area(&info);
>> -    if (offset_in_page(addr)) {
>> +printk("io_uring_mmu_get_unmapped_area() address 1 is: %px\n", addr);
>> +    addr = NULL;
>> +    if (!addr) {
>>          info.flags = 0;
>>          info.low_limit = TASK_UNMAPPED_BASE;
>>          info.high_limit = mmap_end;
>>          addr = vm_unmapped_area(&info);
>>      }
>> +printk("io_uring_mmu_get_unmapped_area() returns address %px\n", addr);
>>
>>      return addr;
>>  }
>>
>>
>> Another option is to disable the call to io_uring_nommu_get_unmapped_area())
>> with the next patch. Maybe you could add printks() to ia64's arch_get_unmapped_area()
>> and check what it returns there?
>>
>> @@ -3654,6 +3658,8 @@ static const struct file_operations io_uring_fops = {
>>  #ifndef CONFIG_MMU
>>      .get_unmapped_area = io_uring_nommu_get_unmapped_area,
>>      .mmap_capabilities = io_uring_nommu_mmap_capabilities,
>> +#elif 0    /* IS_ENABLED(CONFIG_IA64) */
>> +    .get_unmapped_area = NULL,
>>  #else
>>      .get_unmapped_area = io_uring_mmu_get_unmapped_area,
>>  #endif
>>
>> Helge
>
> Thanks Helge.  Sample output from that first patch:
>
> [Wed Jul 12 13:09:50 2023] io_uring_mmu_get_unmapped_area() address 1 is: 1ffffffffff40000
> [Wed Jul 12 13:09:50 2023] io_uring_mmu_get_unmapped_area() returns address 2000000001e40000
> [Wed Jul 12 13:09:50 2023] io_uring_mmu_get_unmapped_area() address 1 is: 1ffffffffff20000
> [Wed Jul 12 13:09:50 2023] io_uring_mmu_get_unmapped_area() returns address 2000000001f20000
> [Wed Jul 12 13:09:50 2023] io_uring_mmu_get_unmapped_area() address 1 is: 1ffffffffff30000
> [Wed Jul 12 13:09:50 2023] io_uring_mmu_get_unmapped_area() returns address 2000000001f30000
> [Wed Jul 12 13:09:50 2023] io_uring_mmu_get_unmapped_area() address 1 is: 1ffffffffff90000
> [Wed Jul 12 13:09:50 2023] io_uring_mmu_get_unmapped_area() returns address 2000000001f90000
>
> This pattern seems to be pretty stable, I tried instead just directly returning the result of a call to arch_get_unmapped_area() at the end of the function and it seems similar:
>
> [Wed Jul 12 13:27:07 2023] io_uring_mmu_get_unmapped_area() would return address 1ffffffffffd0000
> [Wed Jul 12 13:27:07 2023] but arch_get_unmapped_area() would return address 2000000001f00000
> [Wed Jul 12 13:27:07 2023] io_uring_mmu_get_unmapped_area() would return address 1ffffffffff00000
> [Wed Jul 12 13:27:07 2023] but arch_get_unmapped_area() would return address 1ffffffffff00000
> [Wed Jul 12 13:27:07 2023] io_uring_mmu_get_unmapped_area() would return address 1fffffffffe20000
> [Wed Jul 12 13:27:07 2023] but arch_get_unmapped_area() would return address 2000000002000000
> [Wed Jul 12 13:27:07 2023] io_uring_mmu_get_unmapped_area() would return address 1fffffffffe30000
> [Wed Jul 12 13:27:07 2023] but arch_get_unmapped_area() would return address 2000000002100000
>
> Is that enough of a clue to go on?

SHMLBA on ia64 is 0x100000:
arch/ia64/include/asm/shmparam.h:#define SHMLBA (1024*1024)
but the values returned by io_uring_mmu_get_unmapped_area() does not fullfill this.

So, probably ia64's SHMLBA isn't pulled in correctly in io_uring/io_uring.c.
Check value of this line:
info.align_mask = PAGE_MASK & (SHMLBA - 1UL);

You could also add
#define SHM_COLOUR 0x100000
in front of the
#ifdef SHM_COLOUR
(define SHM_COLOUR in io_uring/kbuf.c too).

Helge