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

[Baoquan He <bhe@redhat.com>]

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>
> 
> 
> ===============================================
> [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