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move the capability checks from sget_userns() to legacy_get_tree()

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1) all call chains leading to sget_userns() pass through ->mount()
instances.
2) none of ->mount() instances is ever called directly - the only
call site is legacy_get_tree()
3) all remaining ->mount() instances end up calling sget_userns()

IOW, we might as well do the capability checks just before calling
->mount().  As for the arguments passed to mount_capable(),
in case of call chains to sget_userns() going through sget(),
we either don't call mount_capable() at all, or pass current_user_ns()
to it.  The call chains going through mount_pseudo_xattr() don't
call mount_capable() at all (SB_KERNMOUNT in flags on those).

That could've been split into smaller steps (lifting the checks
into sget(), then callers of sget(), then all the way to the
entries of every ->mount() out there, then to the sole caller),
but that would be too much churn for little benefit...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Al Viro 2019-05-12 17:09:01 -04:00
parent bb7b6b2bbd
commit 2527b284de
3 changed files with 7 additions and 4 deletions
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5
fs/fs_context.c
View File

@ -662,6 +662,11 @@ static int legacy_get_tree(struct fs_context *fc)
struct super_block *sb;
struct dentry *root;
if (!(fc->sb_flags & (SB_KERNMOUNT|SB_SUBMOUNT))) {
if (!mount_capable(fc->fs_type, current_user_ns()))
return -EPERM;
}
root = fc->fs_type->mount(fc->fs_type, fc->sb_flags,
fc->source, ctx->legacy_data);
if (IS_ERR(root))

2
fs/internal.h
View File

@ -18,6 +18,7 @@ struct path;
struct mount;
struct shrink_control;
struct fs_context;
struct user_namespace;
/*
* block_dev.c
@ -113,6 +114,7 @@ extern struct file *alloc_empty_file_noaccount(int, const struct cred *);
extern int reconfigure_super(struct fs_context *);
extern bool trylock_super(struct super_block *sb);
extern struct super_block *user_get_super(dev_t);
extern bool mount_capable(struct file_system_type *, struct user_namespace *);
/*
* open.c

4
fs/super.c
View File

@ -583,10 +583,6 @@ struct super_block *sget_userns(struct file_system_type *type,
struct super_block *old;
int err;
if (!(flags & (SB_KERNMOUNT|SB_SUBMOUNT))) {
if (!mount_capable(type, user_ns))
return ERR_PTR(-EPERM);
}
retry:
spin_lock(&sb_lock);
if (test) {