Re: [PATCH] fs/proc/kcore: reinstate bounce buffer for KCORE_TEXT regions

[Lorenzo Stoakes <lstoakes@gmail.com>]

On Wed, Aug 02, 2023 at 12:01:16AM +0800, Baoquan He wrote:
> On 08/01/23 at 11:57pm, Baoquan He wrote:
> > On 07/31/23 at 10:50pm, Lorenzo Stoakes wrote:
> > > Some architectures do not populate the entire range categorised by
> > > KCORE_TEXT, so we must ensure that the kernel address we read from is
> > > valid.
> > >
> > > Unfortunately there is no solution currently available to do so with a
> > > purely iterator solution so reinstate the bounce buffer in this instance so
> > > we can use copy_from_kernel_nofault() in order to avoid page faults when
> > > regions are unmapped.
> > >
> > > This change partly reverts commit 2e1c0170771e ("fs/proc/kcore: avoid
> > > bounce buffer for ktext data"), reinstating the bounce buffer, but adapts
> > > the code to continue to use an iterator.
> > >
> > > Fixes: 2e1c0170771e ("fs/proc/kcore: avoid bounce buffer for ktext data")
> > > Reported-by: Jiri Olsa <olsajiri@gmail.com>
> > > Closes: https://lore.kernel.org/all/ZHc2fm+9daF6cgCE@krava
> > > Cc: stable@vger.kernel.org
> > > Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
> > > ---
> > >  fs/proc/kcore.c | 26 +++++++++++++++++++++++++-
> > >  1 file changed, 25 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
> > > index 9cb32e1a78a0..3bc689038232 100644
> > > --- a/fs/proc/kcore.c
> > > +++ b/fs/proc/kcore.c
> > > @@ -309,6 +309,8 @@ static void append_kcore_note(char *notes, size_t *i, const char *name,
> > >
> > >  static ssize_t read_kcore_iter(struct kiocb *iocb, struct iov_iter *iter)
> > >  {
> > > +	struct file *file = iocb->ki_filp;
> > > +	char *buf = file->private_data;
> > >  	loff_t *fpos = &iocb->ki_pos;
> > >  	size_t phdrs_offset, notes_offset, data_offset;
> > >  	size_t page_offline_frozen = 1;
> > > @@ -554,11 +556,22 @@ static ssize_t read_kcore_iter(struct kiocb *iocb, struct iov_iter *iter)
> > >  			fallthrough;
> > >  		case KCORE_VMEMMAP:
> > >  		case KCORE_TEXT:
> > > +			/*
> > > +			 * Sadly we must use a bounce buffer here to be able to
> > > +			 * make use of copy_from_kernel_nofault(), as these
> > > +			 * memory regions might not always be mapped on all
> > > +			 * architectures.
> > > +			 */
> > > +			if (copy_from_kernel_nofault(buf, (void *)start, tsz)) {
> > > +				if (iov_iter_zero(tsz, iter) != tsz) {
> > > +					ret = -EFAULT;
> > > +					goto out;
> > > +				}
> > >  			/*
> > >  			 * We use _copy_to_iter() to bypass usermode hardening
> > >  			 * which would otherwise prevent this operation.
> > >  			 */
> > > -			if (_copy_to_iter((char *)start, tsz, iter) != tsz) {
> > > +			} else if (_copy_to_iter(buf, tsz, iter) != tsz) {
> > >  				ret = -EFAULT;
> > >  				goto out;
> > >  			}
> > > @@ -595,6 +608,10 @@ static int open_kcore(struct inode *inode, struct file *filp)
> > >  	if (ret)
> > >  		return ret;
> > >
> > > +	filp->private_data = kmalloc(PAGE_SIZE, GFP_KERNEL);
> > > +	if (!filp->private_data)
> > > +		return -ENOMEM;
> > > +
> > >  	if (kcore_need_update)
> > >  		kcore_update_ram();
> > >  	if (i_size_read(inode) != proc_root_kcore->size) {
> > > @@ -605,9 +622,16 @@ static int open_kcore(struct inode *inode, struct file *filp)
> > >  	return 0;
> > >  }
> > >
> > > +static int release_kcore(struct inode *inode, struct file *file)
> > > +{
> > > +	kfree(file->private_data);
> > > +	return 0;
> > > +}
> > > +
> > >  static const struct proc_ops kcore_proc_ops = {
> > >  	.proc_read_iter	= read_kcore_iter,
> > >  	.proc_open	= open_kcore,
> > > +	.proc_release	= release_kcore,
> > >  	.proc_lseek	= default_llseek,
> > >  };
> >
> > On 6.5-rc4, the failures can be reproduced stably on a arm64 machine.
> > With patch applied, both makedumpfile and objdump test cases passed.
> >
> > And the code change looks good to me, thanks.
> >
> > Tested-by: Baoquan He <bhe@redhat.com>
> > Reviewed-by: Baoquan He <bhe@redhat.com>

Thanks!

> >
> >
> > ===============================================
> > [root@ ~]# makedumpfile --mem-usage /proc/kcore
> > The kernel version is not supported.
> > The makedumpfile operation may be incomplete.
> >
> > TYPE		PAGES			EXCLUDABLE	DESCRIPTION
> > ----------------------------------------------------------------------
> > ZERO		76234           	yes		Pages filled with zero
> > NON_PRI_CACHE	147613          	yes		Cache pages without private flag
> > PRI_CACHE	3847            	yes		Cache pages with private flag
> > USER		15276           	yes		User process pages
> > FREE		15809884        	yes		Free pages
> > KERN_DATA	459950          	no		Dumpable kernel data
> >
> > page size:		4096
> > Total pages on system:	16512804
> > Total size on system:	67636445184      Byte
> >
> > [root@ ~]# objdump -d  --start-address=0x^C
> > [root@ ~]# cat /proc/kallsyms | grep ksys_read
> > ffffab3be77229d8 T ksys_readahead
> > ffffab3be782a700 T ksys_read
> > [root@ ~]# objdump -d  --start-address=0xffffab3be782a700 --stop-address=0xffffab3be782a710 /proc/kcore
> >
> > /proc/kcore:     file format elf64-littleaarch64
> >
> >
> > Disassembly of section load1:
> >
> > ffffab3be782a700 <load1+0x41a700>:
> > ffffab3be782a700:	aa1e03e9 	mov	x9, x30
> > ffffab3be782a704:	d503201f 	nop
> > ffffab3be782a708:	d503233f 	paciasp
> > ffffab3be782a70c:	a9bc7bfd 	stp	x29, x30, [sp, #-64]!
> > objdump: error: /proc/kcore(load2) is too large (0x7bff70000000 bytes)
> > objdump: Reading section load2 failed because: memory exhausted
>
> By the way, I can still see the objdump error saying kcore is too large
> as above, at the same time there's console printing as below. Haven't
> checked it's objdump's issue or kernel's.
>
> [ 6631.575800] __vm_enough_memory: pid: 5321, comm: objdump, not enough memory for the allocation
> [ 6631.584469] __vm_enough_memory: pid: 5321, comm: objdump, not enough memory for the allocation
>

Yeah this issue existed before this patch was applied on arm64, apparently
an ancient objdump bug according to the other thread [0]. I confirmed it
exists on v6.0 kernel for instance.

[0]:https://lore.kernel.org/all/7b94619ad89c9e308c7aedef2cacfa10b8666e69.camel@gmx.de/