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Kees Cook f6cfc6bcfd exec: Fix ToCToU between perm check and set-uid/gid usage 
commit f50733b45d upstream.

When opening a file for exec via do_filp_open(), permission checking is
done against the file's metadata at that moment, and on success, a file
pointer is passed back. Much later in the execve() code path, the file
metadata (specifically mode, uid, and gid) is used to determine if/how
to set the uid and gid. However, those values may have changed since the
permissions check, meaning the execution may gain unintended privileges.

For example, if a file could change permissions from executable and not
set-id:

---------x 1 root root 16048 Aug  7 13:16 target

to set-id and non-executable:

---S------ 1 root root 16048 Aug  7 13:16 target

it is possible to gain root privileges when execution should have been
disallowed.

While this race condition is rare in real-world scenarios, it has been
observed (and proven exploitable) when package managers are updating
the setuid bits of installed programs. Such files start with being
world-executable but then are adjusted to be group-exec with a set-uid
bit. For example, "chmod o-x,u+s target" makes "target" executable only
by uid "root" and gid "cdrom", while also becoming setuid-root:

-rwxr-xr-x 1 root cdrom 16048 Aug  7 13:16 target

becomes:

-rwsr-xr-- 1 root cdrom 16048 Aug  7 13:16 target

But racing the chmod means users without group "cdrom" membership can
get the permission to execute "target" just before the chmod, and when
the chmod finishes, the exec reaches brpm_fill_uid(), and performs the
setuid to root, violating the expressed authorization of "only cdrom
group members can setuid to root".

Re-check that we still have execute permissions in case the metadata
has changed. It would be better to keep a copy from the perm-check time,
but until we can do that refactoring, the least-bad option is to do a
full inode_permission() call (under inode lock). It is understood that
this is safe against dead-locks, but hardly optimal.

Reported-by: Marco Vanotti <mvanotti@google.com>
Tested-by: Marco Vanotti <mvanotti@google.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: stable@vger.kernel.org
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Signed-off-by: Kees Cook <kees@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2024-08-19 06:00:03 +02:00
arch x86/mtrr: Check if fixed MTRRs exist before saving them 2024-08-14 13:52:59 +02:00
block block/mq-deadline: Fix the tag reservation code 2024-08-14 13:53:02 +02:00
certs certs: Fix build error when PKCS#11 URI contains semicolon 2023-02-09 11:28:11 +01:00
crypto crypto: aead,cipher - zeroize key buffer after use 2024-07-11 12:47:05 +02:00
Documentation smb3: fix setting SecurityFlags when encryption is required 2024-08-14 13:52:59 +02:00
drivers i2c: qcom-geni: fix missing clk_disable_unprepare() and geni_se_resources_off() 2024-08-14 13:53:03 +02:00
fs exec: Fix ToCToU between perm check and set-uid/gid usage 2024-08-19 06:00:03 +02:00
include netfilter: nf_tables: allow clone callbacks to sleep 2024-08-14 13:53:03 +02:00
init rust: SHADOW_CALL_STACK is incompatible with Rust 2024-08-11 12:35:59 +02:00
io_uring io_uring: fix io_match_task must_hold 2024-08-03 08:49:53 +02:00
ipc sysctl: treewide: drop unused argument ctl_table_root::set_ownership(table) 2024-08-11 12:35:51 +02:00
kernel sched/smt: Fix unbalance sched_smt_present dec/inc 2024-08-14 13:53:00 +02:00
lib kobject_uevent: Fix OOB access within zap_modalias_env() 2024-08-03 08:49:39 +02:00
LICENSES LICENSES/LGPL-2.1: Add LGPL-2.1-or-later as valid identifiers 2021-12-16 14:33:10 +01:00
mm mm/hugetlb: fix potential race in __update_and_free_hugetlb_folio() 2024-08-14 13:53:02 +02:00
net mptcp: pm: do not ignore 'subflow' if 'signal' flag is also set 2024-08-19 06:00:02 +02:00
rust rust: macros: fix soundness issue in module! macro 2024-05-17 11:56:17 +02:00
samples work around gcc bugs with 'asm goto' with outputs 2024-02-23 09:12:28 +01:00
scripts kbuild: avoid build error when single DTB is turned into composite DTB 2024-08-03 08:49:48 +02:00
security apparmor: Fix null pointer deref when receiving skb during sock creation 2024-08-03 08:49:50 +02:00
sound ASoC: topology: Fix route memory corruption 2024-08-19 06:00:03 +02:00
tools selftests: mptcp: join: test both signal & subflow 2024-08-19 06:00:02 +02:00
usr usr/gen_init_cpio.c: remove unnecessary -1 values from int file 2022-10-03 14:21:44 -07:00
virt KVM: Fix a data race on last_boosted_vcpu in kvm_vcpu_on_spin() 2024-06-27 13:46:21 +02:00
.clang-format inet: ping: use hlist_nulls rcu iterator during lookup 2022-12-01 12:42:46 +01:00
.cocciconfig
.get_maintainer.ignore get_maintainer: add Alan to .get_maintainer.ignore 2022-08-20 15:17:44 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore Kbuild: add Rust support 2022-09-28 09:02:20 +02:00
.mailmap 9 hotfixes. 6 for MM, 3 for other areas. Four of these patches address 2022-12-10 17:10:52 -08:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: Remove Michal Marek from Kbuild maintainers 2022-11-16 14:53:00 +09:00
Kbuild Kbuild updates for v6.1 2022-10-10 12:00:45 -07:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS MAINTAINERS: add leah to 6.1 MAINTAINERS file 2024-05-17 11:56:16 +02:00
Makefile Linux 6.1.105 2024-08-14 13:53:03 +02:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.