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Subject[PATCH v2] fs: Fix the default values of i_uid/i_gid on /proc/sys inodes.
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This also fixes a problem where, in a user namespace without root user
mapping, it is not possible to write to /proc/sys/kernel/shmmax.

The problem was introduced by the combination of the two commits:
* 81754357770ebd900801231e7bc8d151ddc00498: fs: Update
i_[ug]id_(read|write) to translate relative to s_user_ns
- this caused the kernel to write INVALID_[UG]ID to i_uid/i_gid
members of /proc/sys inodes if a containing userns does not have
entries for root in the uid/gid_map.
* 0bd23d09b874e53bd1a2fe2296030aa2720d7b08: vfs: Don't modify inodes
with a uid or gid unknown to the vfs
- changed the kernel to prevent opens for write if the i_uid/i_gid
field in the inode is invalid

This commit fixes the issue by defaulting i_uid/i_gid to
GLOBAL_ROOT_UID/GID. Note that these values are not used for /proc/sys
access checks, so the change does not otherwise affect /proc semantics.

Tested: Used a repro program that creates a user namespace without any
mapping and stat'ed /proc/$PID/root/proc/sys/kernel/shmmax from outside.
Before the change, it shows the overflow uid, with the change it's 0.

Signed-off-by: Radoslaw Burny <rburny@google.com>
---
Changelog since v1:
- Updated the commit title and description.

fs/proc/proc_sysctl.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index c74570736b24..36ad1b0d6259 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -499,6 +499,10 @@ static struct inode *proc_sys_make_inode(struct super_block *sb,

if (root->set_ownership)
root->set_ownership(head, table, &inode->i_uid, &inode->i_gid);
+ else {
+ inode->i_uid = GLOBAL_ROOT_UID;
+ inode->i_gid = GLOBAL_ROOT_GID;
+ }

return inode;
}
--
2.22.0.410.gd8fdbe21b5-goog
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