Re: [PATCH v1] proc/vmcore: fix clearing user buffer by properly using clear_user()

[Baoquan He]

On 11/11/21 at 08:18pm, David Hildenbrand wrote:
> To clear a user buffer we cannot simply use memset, we have to use
> clear_user(). Using a kernel config based on rawhide Fedora and a
> virtio-mem device that registers a vmcore_cb, I can easily trigger:
> 
> [   11.327580] systemd[1]: Starting Kdump Vmcore Save Service...
> [   11.339697] kdump[420]: Kdump is using the default log level(3).
> [   11.370964] kdump[453]: saving to /sysroot/var/crash/127.0.0.1-2021-11-11-14:59:22/
> [   11.373997] kdump[458]: saving vmcore-dmesg.txt to /sysroot/var/crash/127.0.0.1-2021-11-11-14:59:22/
> [   11.385357] kdump[465]: saving vmcore-dmesg.txt complete
> [   11.386722] kdump[467]: saving vmcore
> [   16.531275] BUG: unable to handle page fault for address: 00007f2374e01000
> [   16.531705] #PF: supervisor write access in kernel mode
> [   16.532037] #PF: error_code(0x0003) - permissions violation
> [   16.532396] PGD 7a523067 P4D 7a523067 PUD 7a528067 PMD 7a525067 PTE 800000007048f867
> [   16.532872] Oops: 0003 [#1] PREEMPT SMP NOPTI
> [   16.533154] CPU: 0 PID: 468 Comm: cp Not tainted 5.15.0+ #6
> [   16.533513] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.14.0-27-g64f37cc530f1-prebuilt.qemu.org 04/01/2014
> [   16.534198] RIP: 0010:read_from_oldmem.part.0.cold+0x1d/0x86
> [   16.534552] Code: ff ff ff e8 05 ff fe ff e9 b9 e9 7f ff 48 89 de 48 c7 c7 38 3b 60 82 e8 f1 fe fe ff 83 fd 08 72 3c 49 8d 7d 08 4c 89 e9 89 e8 <49> c7 45 00 00 00 00 00 49 c7 44 05 f8 00 00 00 00 48 83 e7 f81
> [   16.535670] RSP: 0018:ffffc9000073be08 EFLAGS: 00010212
> [   16.535998] RAX: 0000000000001000 RBX: 00000000002fd000 RCX: 00007f2374e01000
> [   16.536441] RDX: 0000000000000001 RSI: 00000000ffffdfff RDI: 00007f2374e01008
> [   16.536878] RBP: 0000000000001000 R08: 0000000000000000 R09: ffffc9000073bc50
> [   16.537315] R10: ffffc9000073bc48 R11: ffffffff829461a8 R12: 000000000000f000
> [   16.537755] R13: 00007f2374e01000 R14: 0000000000000000 R15: ffff88807bd421e8
> [   16.538200] FS:  00007f2374e12140(0000) GS:ffff88807f000000(0000) knlGS:0000000000000000
> [   16.538696] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [   16.539055] CR2: 00007f2374e01000 CR3: 000000007a4aa000 CR4: 0000000000350eb0
> [   16.539510] Call Trace:
> [   16.539679]  <TASK>
> [   16.539828]  read_vmcore+0x236/0x2c0
> [   16.540063]  ? enqueue_hrtimer+0x2f/0x80
> [   16.540323]  ? inode_security+0x22/0x60
> [   16.540572]  proc_reg_read+0x55/0xa0
> [   16.540807]  vfs_read+0x95/0x190
> [   16.541022]  ksys_read+0x4f/0xc0
> [   16.541238]  do_syscall_64+0x3b/0x90
> [   16.541475]  entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> To fix, properly use clear_user() when required.

Looks a great fix to me, thanks for fixing this. 

Check the code, clear_user invokes access_ok to do check, then call
memset(). It's unclear to me how the bug is triggered, could you
please tell more so that I can learn? 


> 
> Fixes: 997c136f518c ("fs/proc/vmcore.c: add hook to read_from_oldmem() to check for non-ram pages")
> Cc: Dave Young <dyoung@xxxxxxxxxx>
> Cc: Baoquan He <bhe@xxxxxxxxxx>
> Cc: Vivek Goyal <vgoyal@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Philipp Rudo <prudo@xxxxxxxxxx>
> Cc: kexec@xxxxxxxxxxxxxxxxxxx
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>  fs/proc/vmcore.c | 20 ++++++++++++--------
>  1 file changed, 12 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/proc/vmcore.c b/fs/proc/vmcore.c
> index 30a3b66f475a..509f85148fee 100644
> --- a/fs/proc/vmcore.c
> +++ b/fs/proc/vmcore.c
> @@ -154,9 +154,13 @@ ssize_t read_from_oldmem(char *buf, size_t count,
>  			nr_bytes = count;
>  
>  		/* If pfn is not ram, return zeros for sparse dump files */
> -		if (!pfn_is_ram(pfn))
> -			memset(buf, 0, nr_bytes);
> -		else {
> +		if (!pfn_is_ram(pfn)) {
> +			tmp = 0;
> +			if (!userbuf)
> +				memset(buf, 0, nr_bytes);
> +			else if (clear_user(buf, nr_bytes))
> +				tmp = -EFAULT;
> +		} else {
>  			if (encrypted)
>  				tmp = copy_oldmem_page_encrypted(pfn, buf,
>  								 nr_bytes,
> @@ -165,12 +169,12 @@ ssize_t read_from_oldmem(char *buf, size_t count,
>  			else
>  				tmp = copy_oldmem_page(pfn, buf, nr_bytes,
>  						       offset, userbuf);
> -
> -			if (tmp < 0) {
> -				up_read(&vmcore_cb_rwsem);
> -				return tmp;
> -			}
>  		}
> +		if (tmp < 0) {
> +			up_read(&vmcore_cb_rwsem);
> +			return tmp;
> +		}
> +
>  		*ppos += nr_bytes;
>  		count -= nr_bytes;
>  		buf += nr_bytes;
> 
> base-commit: debe436e77c72fcee804fb867f275e6d31aa999c
> -- 
> 2.31.1
>