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

Baoquan He
2023-07-21T13:49:39Z

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.

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
> >
>