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userns: move user access out of the mutex

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The old code would hold the userns_state_mutex indefinitely if
memdup_user_nul stalled due to e.g. a userfault region. Prevent that by
moving the memdup_user_nul in front of the mutex_lock().

Note: This changes the error precedence of invalid buf/count/*ppos vs
map already written / capabilities missing.

Fixes: 22d917d80e ("userns: Rework the user_namespace adding uid/gid...")
Cc: stable@vger.kernel.org
Signed-off-by: Jann Horn <jannh@google.com>
Acked-by: Christian Brauner <christian@brauner.io>
Acked-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
This commit is contained in:
Jann Horn 2018-06-25 18:34:19 +02:00 committed by Eric W. Biederman
parent 355139a8db
commit 5820f140ed
1 changed files with 10 additions and 14 deletions
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24
kernel/user_namespace.c
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@ -859,7 +859,16 @@ static ssize_t map_write(struct file *file, const char __user *buf,
unsigned idx;
struct uid_gid_extent extent;
char *kbuf = NULL, *pos, *next_line;
ssize_t ret = -EINVAL;
ssize_t ret;
/* Only allow < page size writes at the beginning of the file */
if ((*ppos != 0) || (count >= PAGE_SIZE))
return -EINVAL;
/* Slurp in the user data */
kbuf = memdup_user_nul(buf, count);
if (IS_ERR(kbuf))
return PTR_ERR(kbuf);
/*
* The userns_state_mutex serializes all writes to any given map.
@ -895,19 +904,6 @@ static ssize_t map_write(struct file *file, const char __user *buf,
if (cap_valid(cap_setid) && !file_ns_capable(file, ns, CAP_SYS_ADMIN))
goto out;
/* Only allow < page size writes at the beginning of the file */
ret = -EINVAL;
if ((*ppos != 0) || (count >= PAGE_SIZE))
goto out;
/* Slurp in the user data */
kbuf = memdup_user_nul(buf, count);
if (IS_ERR(kbuf)) {
ret = PTR_ERR(kbuf);
kbuf = NULL;
goto out;
}
/* Parse the user data */
ret = -EINVAL;
pos = kbuf;