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proc: handle subset=pid separately in userns visibility checks

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When procfs is mounted with subset=pid, only the dynamic process-related
part of the filesystem remains visible. That part cannot be hidden by
overmounts, so checking whether an existing procfs mount is fully
visible does not make sense for this mode.

At the same time, a subset=pid procfs mount must not be used as evidence
that a later procfs mount would not reveal additional information. It
provides a restricted view of procfs, not the full filesystem view.

Mark subset=pid procfs instances as restricted variants. Ignore
restricted variants when looking for an already-visible mount, and allow
new restricted variants without consulting mnt_already_visible().

Signed-off-by: Alexey Gladkov <legion@kernel.org>
Link: https://patch.msgid.link/4d5e760c3d534dd2e05578d119cc408450053a98.1777278334.git.legion@kernel.org
Reviewed-by: Aleksa Sarai <aleksa@amutable.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Alexey Gladkov
2026-04-27 10:26:07 +02:00
committed by Christian Brauner
parent 1991a8f693
commit 05dab768fc
3 changed files with 20 additions and 1 deletions
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17
fs/namespace.c
View File

@@ -6353,10 +6353,18 @@ static bool mnt_already_visible(struct mnt_namespace *ns,
guard(namespace_shared)();
hlist_for_each_entry(mnt, &ns->mnt_visible_mounts, mnt_ns_visible) {
const struct super_block *sb_visible = mnt->mnt.mnt_sb;
struct mount *child;
int mnt_flags;
if (mnt->mnt.mnt_sb->s_type != sb->s_type)
if (sb_visible->s_type != sb->s_type)
continue;
/*
* Restricted variants are not compatible with anything, even
* other restricted variants.
*/
if (sb_visible->s_iflags & SB_I_RESTRICTED_VARIANT)
continue;
/* A local view of the mount flags */
@@ -6418,6 +6426,13 @@ static bool mount_too_revealing(const struct super_block *sb, int *new_mnt_flags
return true;
}
/*
* Restricted variants don't need an already visible mount because they
* don't expose the full filesystem view.
*/
if (s_iflags & SB_I_RESTRICTED_VARIANT)
return false;
return !mnt_already_visible(ns, sb, new_mnt_flags);
}

3
fs/proc/root.c
View File

@@ -275,6 +275,9 @@ static int proc_fill_super(struct super_block *s, struct fs_context *fc)
s->s_time_gran = 1;
s->s_fs_info = fs_info;
if (fs_info->pidonly == PROC_PIDONLY_ON)
s->s_iflags |= SB_I_RESTRICTED_VARIANT;
/*
* procfs isn't actually a stacking filesystem; however, there is
* too much magic going on inside it to permit stacking things on

1
include/linux/fs/super_types.h
View File

@@ -326,6 +326,7 @@ struct super_block {
#define SB_I_STABLE_WRITES 0x00000008 /* don't modify blks until WB is done */
/* sb->s_iflags to limit user namespace mounts */
#define SB_I_RESTRICTED_VARIANT 0x00000010
#define SB_I_IMA_UNVERIFIABLE_SIGNATURE 0x00000020
#define SB_I_UNTRUSTED_MOUNTER 0x00000040
#define SB_I_EVM_HMAC_UNSUPPORTED 0x00000080