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

Helge Deller
2023-07-12T16:25:20Z

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