Re: [PATCH v8 1/4] fs/proc/kcore: avoid bounce buffer for ktext data

Jiri Olsa
2023-07-21T14:14:03Z

On Fri, Jul 21, 2023 at 09:48:37PM +0800, Baoquan He wrote:
> Hi Jiri,
>
> On 05/31/23 at 01:58pm, Jiri Olsa wrote:
> > On Thu, Mar 23, 2023 at 10:15:16AM +0000, Lorenzo Stoakes wrote:
> > > Commit df04abfd181a ("fs/proc/kcore.c: Add bounce buffer for ktext data")
> > > introduced the use of a bounce buffer to retrieve kernel text data for
> > > /proc/kcore in order to avoid failures arising from hardened user copies
> > > enabled by CONFIG_HARDENED_USERCOPY in check_kernel_text_object().
> > >
> > > We can avoid doing this if instead of copy_to_user() we use _copy_to_user()
> > > which bypasses the hardening check. This is more efficient than using a
> > > bounce buffer and simplifies the code.
> > >
> > > We do so as part an overall effort to eliminate bounce buffer usage in the
> > > function with an eye to converting it an iterator read.
> > >
> > > Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
> > > Reviewed-by: David Hildenbrand <david@redhat.com>
> >
> > hi,
> > sorry for late feedback, but looks like this one breaks reading
> > /proc/kcore with objdump for me:
> >
> > # cat /proc/kallsyms | grep ksys_read
> > ffffffff8150ebc0 T ksys_read
> > # objdump -d --start-address=0xffffffff8150ebc0 --stop-address=0xffffffff8150ebd0 /proc/kcore
> >
> > /proc/kcore: file format elf64-x86-64
> >
> > objdump: Reading section load1 failed because: Bad address
> >
> > reverting this makes it work again
>
> I met this too when I executed below command to trigger a kcore reading.
> I wanted to do a simple testing during system running and got this.
>
> makedumpfile --mem-usage /proc/kcore
>
> Later I tried your above objdump testing, it corrupted system too.
>
> Is there any conclusion about this issue you reported? I could miss
> things in the discussion or patch posting to fix this.

hi,
thanks for your reply, I meant to ping on this again

AFAIK there was no answer yet.. I managed to cleanly revert the patch when
I needed the functionality, then got sidetracked and forgot about this

I just re-tested and it's still failing for me, would be great to get it fixed

Lorenzo, any idea?

thanks,
jirka

>
> Thanks
> Baoquan
>
> >
> >
> > > ---
> > > fs/proc/kcore.c | 17 +++++------------
> > > 1 file changed, 5 insertions(+), 12 deletions(-)
> > >
> > > diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
> > > index 71157ee35c1a..556f310d6aa4 100644
> > > --- a/fs/proc/kcore.c
> > > +++ b/fs/proc/kcore.c
> > > @@ -541,19 +541,12 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
> > > case KCORE_VMEMMAP:
> > > case KCORE_TEXT:
> > > /*
> > > - * Using bounce buffer to bypass the
> > > - * hardened user copy kernel text checks.
> > > + * We use _copy_to_user() to bypass usermode hardening
> > > + * which would otherwise prevent this operation.
> > > */
> > > - if (copy_from_kernel_nofault(buf, (void *)start, tsz)) {
> > > - if (clear_user(buffer, tsz)) {
> > > - ret = -EFAULT;
> > > - goto out;
> > > - }
> > > - } else {
> > > - if (copy_to_user(buffer, buf, tsz)) {
> > > - ret = -EFAULT;
> > > - goto out;
> > > - }
> > > + if (_copy_to_user(buffer, (char *)start, tsz)) {
> > > + ret = -EFAULT;
> > > + goto out;
> > > }
> > > break;
> > > default:
> > > --
> > > 2.39.2
> > >
> >
>