On Thu, 20 Jul 2023, Oleksandr Tymoshenko wrote:
> User extended attributes are not enabled in tmpfs because
> the size of the value is not limited and the memory allocated
> for it is not counted against any limit. Malicious
> non-privileged user can exhaust kernel memory by creating
> user.* extended attribute with very large value.
>
> There are still situations when enabling suport for extended
> user attributes on tmpfs is required and the attack vector
> is not applicable, for instance batch jobs with trusted binaries.
>
> This patch introduces two mount options to enable/disable
> support for user.* extended attributes on tmpfs:
>
> user_xattr enable support for user extended aatributes
> nouser_xattr disable support for user extended attributes
>
> The default behavior of the filesystem is not changed.
>
> Signed-off-by: Oleksandr Tymoshenko <ovt@xxxxxxxxxx>
Thanks, but no.
This is not something we want mount options for:
we just want to limit the memory usage of tmpfs user xattrs.
I've had the patch to do that limiting (taking it out of the inode
space already limited by nr_inodes) in my test tree for 2.5 years now:
waiting to reach the top of the heap to pull together and submit.
Your sending this patch does help to raise the priority for my
sending that patch: thank you; but I cannot promise when that will be.
(And the way mm/shmem.c is conflicted between vfs and mm trees
is rather discouraging development there at the moment: I'm hoping
it can be mostly wrested back into the mm tree in the next cycle.)
Hugh
> ---
> Documentation/filesystems/tmpfs.rst | 12 ++++++++
> include/linux/shmem_fs.h | 1 +
> mm/shmem.c | 45 +++++++++++++++++++++++++++++
> 3 files changed, 58 insertions(+)
>
> diff --git a/Documentation/filesystems/tmpfs.rst b/Documentation/filesystems/tmpfs.rst
> index f18f46be5c0c..5700ba72d095 100644
> --- a/Documentation/filesystems/tmpfs.rst
> +++ b/Documentation/filesystems/tmpfs.rst
> @@ -215,6 +215,16 @@ will give you tmpfs instance on /mytmpfs which can allocate 10GB
> RAM/SWAP in 10240 inodes and it is only accessible by root.
>
>
> +tmpfs, when compiled with CONFIG_TMPFS_XATTR, does not support
> +Extended User Attributes for security reasons. The support can be
> +enabled/disabled by two mount options:
> +
> +============ ===========================================
> +user_xattr Enable support for Extended User Attributes
> +nouser_xattr Disable upport for Extended User Attributes
> +============ ===========================================
> +
> +
> :Author:
> Christoph Rohland <cr@xxxxxxx>, 1.12.01
> :Updated:
> @@ -223,3 +233,5 @@ RAM/SWAP in 10240 inodes and it is only accessible by root.
> KOSAKI Motohiro, 16 Mar 2010
> :Updated:
> Chris Down, 13 July 2020
> +:Updated:
> + Oleksandr Tymoshenko, 19 July 2023
> diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
> index 9029abd29b1c..f06d18b9041c 100644
> --- a/include/linux/shmem_fs.h
> +++ b/include/linux/shmem_fs.h
> @@ -53,6 +53,7 @@ struct shmem_sb_info {
> spinlock_t shrinklist_lock; /* Protects shrinklist */
> struct list_head shrinklist; /* List of shinkable inodes */
> unsigned long shrinklist_len; /* Length of shrinklist */
> + bool user_xattr; /* user.* xattrs are allowed */
> };
>
> static inline struct shmem_inode_info *SHMEM_I(struct inode *inode)
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 2f2e0e618072..4f7d46d65494 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -85,6 +85,7 @@ static struct vfsmount *shm_mnt;
>
> #define BLOCKS_PER_PAGE (PAGE_SIZE/512)
> #define VM_ACCT(size) (PAGE_ALIGN(size) >> PAGE_SHIFT)
> +#define TMPFS_USER_XATTR_INDEX 1
>
> /* Pretend that each entry is of this size in directory's i_size */
> #define BOGO_DIRENT_SIZE 20
> @@ -116,11 +117,13 @@ struct shmem_options {
> int huge;
> int seen;
> bool noswap;
> + bool user_xattr;
> #define SHMEM_SEEN_BLOCKS 1
> #define SHMEM_SEEN_INODES 2
> #define SHMEM_SEEN_HUGE 4
> #define SHMEM_SEEN_INUMS 8
> #define SHMEM_SEEN_NOSWAP 16
> +#define SHMEM_SEEN_USER_XATTR 32
> };
>
> #ifdef CONFIG_TMPFS
> @@ -3447,6 +3450,16 @@ static int shmem_xattr_handler_get(const struct xattr_handler *handler,
> const char *name, void *buffer, size_t size)
> {
> struct shmem_inode_info *info = SHMEM_I(inode);
> + struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
> +
> + switch (handler->flags) {
> + case TMPFS_USER_XATTR_INDEX:
> + if (!sbinfo->user_xattr)
> + return -EOPNOTSUPP;
> + break;
> + default:
> + break;
> + }
>
> name = xattr_full_name(handler, name);
> return simple_xattr_get(&info->xattrs, name, buffer, size);
> @@ -3459,8 +3472,18 @@ static int shmem_xattr_handler_set(const struct xattr_handler *handler,
> size_t size, int flags)
> {
> struct shmem_inode_info *info = SHMEM_I(inode);
> + struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
> int err;
>
> + switch (handler->flags) {
> + case TMPFS_USER_XATTR_INDEX:
> + if (!sbinfo->user_xattr)
> + return -EOPNOTSUPP;
> + break;
> + default:
> + break;
> + }
> +
> name = xattr_full_name(handler, name);
> err = simple_xattr_set(&info->xattrs, name, value, size, flags, NULL);
> if (!err) {
> @@ -3482,9 +3505,17 @@ static const struct xattr_handler shmem_trusted_xattr_handler = {
> .set = shmem_xattr_handler_set,
> };
>
> +static const struct xattr_handler shmem_user_xattr_handler = {
> + .prefix = XATTR_USER_PREFIX,
> + .flags = TMPFS_USER_XATTR_INDEX,
> + .get = shmem_xattr_handler_get,
> + .set = shmem_xattr_handler_set,
> +};
> +
> static const struct xattr_handler *shmem_xattr_handlers[] = {
> &shmem_security_xattr_handler,
> &shmem_trusted_xattr_handler,
> + &shmem_user_xattr_handler,
> NULL
> };
>
> @@ -3604,6 +3635,8 @@ enum shmem_param {
> Opt_inode32,
> Opt_inode64,
> Opt_noswap,
> + Opt_user_xattr,
> + Opt_nouser_xattr,
> };
>
> static const struct constant_table shmem_param_enums_huge[] = {
> @@ -3626,6 +3659,8 @@ const struct fs_parameter_spec shmem_fs_parameters[] = {
> fsparam_flag ("inode32", Opt_inode32),
> fsparam_flag ("inode64", Opt_inode64),
> fsparam_flag ("noswap", Opt_noswap),
> + fsparam_flag ("user_xattr", Opt_user_xattr),
> + fsparam_flag ("nouser_xattr", Opt_nouser_xattr),
> {}
> };
>
> @@ -3717,6 +3752,14 @@ static int shmem_parse_one(struct fs_context *fc, struct fs_parameter *param)
> ctx->noswap = true;
> ctx->seen |= SHMEM_SEEN_NOSWAP;
> break;
> + case Opt_user_xattr:
> + ctx->user_xattr = true;
> + ctx->seen |= SHMEM_SEEN_USER_XATTR;
> + break;
> + case Opt_nouser_xattr:
> + ctx->user_xattr = false;
> + ctx->seen |= SHMEM_SEEN_USER_XATTR;
> + break;
> }
> return 0;
>
> @@ -3834,6 +3877,8 @@ static int shmem_reconfigure(struct fs_context *fc)
> sbinfo->max_inodes = ctx->inodes;
> sbinfo->free_inodes = ctx->inodes - inodes;
> }
> + if (ctx->seen & SHMEM_SEEN_USER_XATTR)
> + sbinfo->user_xattr = ctx->user_xattr;
>
> /*
> * Preserve previous mempolicy unless mpol remount option was specified.
> --
> 2.41.0.255.g8b1d071c50-goog
>
>
