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

[Baoquan He <bhe@redhat.com>]

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