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

[Baoquan He]

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@xxxxxxxxx>
> > Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
> 
> 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
> > 
>