[Virtio-fs] Problem using gdb/lldb on a binary residing on a DAX-enabled volume

[Vivek Goyal]

On Tue, Aug 24, 2021 at 02:04:45PM +0900, Chirantan Ekbote wrote:
> Hi Sergio,
> 
> On Mon, Aug 23, 2021 at 6:31 PM Sergio Lopez <slp at redhat.com> wrote:
> >
> > Hi,
> >
> > I've noticed that trying to use gdb/lldb on any binary residing on a
> > DAX-enabled virtio-fs volume leads to a SIGSEGV in userspace...
> >
> > Seems like DAX breaks something in the ptrace_access_vm path. On a
> > volume without DAX works fine.
> >
> 
> We've seen this as well and unfortunately it doesn't appear to be
> limited to virtio-fs.  Using DAX on a ext4 formatted virtio-pmem disk
> image has the same problem.  We've actually disabled DAX everywhere
> because of this.

Thanks Chirantan for confirming that this probably a generic DAX
issue (and not limited to virtiofs) and providing all the details.

Copying Dan Williams. He might have an idea.

Vivek

> 
> Unfortunately most of the details are in an internal bug report but
> I'll try to extract the relevant bits here.  This is well outside my
> depth so I've CC'd some of the people who have looked at this.  The
> initial bug report was for virtio-pmem+ext4 so some of the details are
> specific to pmem but I suspect something similar is happening for
> virtio-fs as well.
> 
> The issue is that process_vm_readv() corrupts the memory of files that
> are mmap()'d when DAX is enabled.
> 
> 1. A filesystem is mounted with DAX enabled.  pmem_attach_disk() sets
> pfn_flags to PFN_DEV|PFN_MAP.  In the fuse case, this appears to
> happen here [1].
> 2. When the (strace/gdb/etc) process does its initial read of the
> mmap()'d region, the pfn flags for the page are inherited from the
> pmem structure set to PFN_DEV|PFN_MAP in step 1.  During a call to
> insert_pfn(), pte_mkdevmap is called to mark the pte as devmap.
> 3. If you follow the ftrace of the process_vm_readv(), it eventually
> reaches do_wp_page(). If the target process had not previously read
> the page in, this would not call do_wp_page() and instead just fault
> in the page normally through the ext4/dax logic.
> 4. do_wp_page() calls vm_normal_page() which returns NULL due to the
> remote pte being marked special and devmap (from above).  If we just
> ignore the devmap check and return the page that has been found and
> allow the normal copy to occur, then no problem occurs.  However, that
> can't be safely done in normal dax cases.  Due to vm_normal_page()
> returning NULL, wp_page_copy() is called (first call site) with a null
> vmf->address.  If the mmap()d file is originally from a non-dax
> filesystem (eg tmpfs), the second wp_page_copy() ends up being called
> with a valid vmf->address.
> 5. cow_user_page() ends up in this unexpected case since
> src==vmf->address is NULL, delimited with the following comment:
> 
>         /*
>          * If the source page was a PFN mapping, we don't have
>          * a "struct page" for it. We do a best-effort copy by
>          * just copying from the original user address. If that
>          * fails, we just zero-fill it. Live with it.
>          */
> 
> The end effect of this is that there is the
> __copy_from_user_inatomic() call with an invalid uaddr because the
> uaddr is from the remote address space.   This results in another page
> fault because that remote address isn't valid in the process calling
> process_vm_readv().  It seems that there's a few issues here, a) that
> it's trying to read from the remote address space as if it were local,
> and b) that the failure here is corrupting the remote processes memory
> and not just returning an empty page which would be less broken.
> 
> In the good case of the mmap()d file being from tmpfs,
> src==vmf->address is non-NULL and copy_user_highpage can properly do
> the copy of the page.  At that point, the caller is able to copy data
> from that page to its own local buffer and return data successfully,
> as well as avoid corrupting the remote process.
> 
> We've also found that reverting "17839856fd58: gup: document and work
> around "COW can break either way" issue" seems to make the problem go
> away.
> 
> Chirantan
> 
> [1]: https://kernel.googlesource.com/pub/scm/linux/kernel/git/torvalds/linux/+/d5ae8d7f85b7f6f6e60f1af8ff4be52b0926fde1/fs/fuse/virtio_fs.c#741
> 
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