[linux] 01/01: ucount: Remove the atomicity from ucount->count (CVE-2017-6874)
debian-kernel at lists.debian.org
debian-kernel at lists.debian.org
Tue Mar 14 21:40:06 UTC 2017
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benh pushed a commit to branch sid
in repository linux.
commit 8851d0b7ac8ca15fede0184bd37ee32dde4d9490
Author: Ben Hutchings <ben at decadent.org.uk>
Date: Tue Mar 14 21:39:16 2017 +0000
ucount: Remove the atomicity from ucount->count (CVE-2017-6874)
...and avoid an ABI change.
---
debian/changelog | 2 +
...nt-remove-the-atomicity-from-ucount-count.patch | 83 ++++++++++++++++++++++
...ns-avoid-abi-change-for-cve-2017-6874-fix.patch | 23 ++++++
debian/patches/series | 2 +
4 files changed, 110 insertions(+)
diff --git a/debian/changelog b/debian/changelog
index d836abf..050fd60 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -145,6 +145,8 @@ linux (4.9.14-1) UNRELEASED; urgency=medium
* tty: n_hdlc: get rid of racy n_hdlc.tbuf (CVE-2017-2636)
* [rt] Refresh one patch that had a textual conflict with 4.9.14
* Ignore ABI changes in rds and ufshcd, not useful to OOT modules
+ * ucount: Remove the atomicity from ucount->count (CVE-2017-6874)
+ * userns: Avoid ABI change for CVE-2017-6874 fix
[ Salvatore Bonaccorso ]
* ACPI / EC: Use busy polling mode when GPE is not enabled.
diff --git a/debian/patches/bugfix/all/ucount-remove-the-atomicity-from-ucount-count.patch b/debian/patches/bugfix/all/ucount-remove-the-atomicity-from-ucount-count.patch
new file mode 100644
index 0000000..0a40cfb
--- /dev/null
+++ b/debian/patches/bugfix/all/ucount-remove-the-atomicity-from-ucount-count.patch
@@ -0,0 +1,83 @@
+From: "Eric W. Biederman" <ebiederm at xmission.com>
+Date: Sun, 5 Mar 2017 15:03:22 -0600
+Subject: ucount: Remove the atomicity from ucount->count
+Origin: https://git.kernel.org/linus/040757f738e13caaa9c5078bca79aa97e11dde88
+Bug-Debian-Security: https://security-tracker.debian.org/tracker/CVE-2017-6874
+
+Always increment/decrement ucount->count under the ucounts_lock. The
+increments are there already and moving the decrements there means the
+locking logic of the code is simpler. This simplification in the
+locking logic fixes a race between put_ucounts and get_ucounts that
+could result in a use-after-free because the count could go zero then
+be found by get_ucounts and then be freed by put_ucounts.
+
+A bug presumably this one was found by a combination of syzkaller and
+KASAN. JongWhan Kim reported the syzkaller failure and Dmitry Vyukov
+spotted the race in the code.
+
+Cc: stable at vger.kernel.org
+Fixes: f6b2db1a3e8d ("userns: Make the count of user namespaces per user")
+Reported-by: JongHwan Kim <zzoru007 at gmail.com>
+Reported-by: Dmitry Vyukov <dvyukov at google.com>
+Reviewed-by: Andrei Vagin <avagin at gmail.com>
+Signed-off-by: "Eric W. Biederman" <ebiederm at xmission.com>
+---
+ include/linux/user_namespace.h | 2 +-
+ kernel/ucount.c | 18 +++++++++++-------
+ 2 files changed, 12 insertions(+), 8 deletions(-)
+
+--- a/include/linux/user_namespace.h
++++ b/include/linux/user_namespace.h
+@@ -65,7 +65,7 @@ struct ucounts {
+ struct hlist_node node;
+ struct user_namespace *ns;
+ kuid_t uid;
+- atomic_t count;
++ int count;
+ atomic_t ucount[UCOUNT_COUNTS];
+ };
+
+--- a/kernel/ucount.c
++++ b/kernel/ucount.c
+@@ -139,7 +139,7 @@ static struct ucounts *get_ucounts(struc
+
+ new->ns = ns;
+ new->uid = uid;
+- atomic_set(&new->count, 0);
++ new->count = 0;
+
+ spin_lock_irq(&ucounts_lock);
+ ucounts = find_ucounts(ns, uid, hashent);
+@@ -150,8 +150,10 @@ static struct ucounts *get_ucounts(struc
+ ucounts = new;
+ }
+ }
+- if (!atomic_add_unless(&ucounts->count, 1, INT_MAX))
++ if (ucounts->count == INT_MAX)
+ ucounts = NULL;
++ else
++ ucounts->count += 1;
+ spin_unlock_irq(&ucounts_lock);
+ return ucounts;
+ }
+@@ -160,13 +162,15 @@ static void put_ucounts(struct ucounts *
+ {
+ unsigned long flags;
+
+- if (atomic_dec_and_test(&ucounts->count)) {
+- spin_lock_irqsave(&ucounts_lock, flags);
++ spin_lock_irqsave(&ucounts_lock, flags);
++ ucounts->count -= 1;
++ if (!ucounts->count)
+ hlist_del_init(&ucounts->node);
+- spin_unlock_irqrestore(&ucounts_lock, flags);
++ else
++ ucounts = NULL;
++ spin_unlock_irqrestore(&ucounts_lock, flags);
+
+- kfree(ucounts);
+- }
++ kfree(ucounts);
+ }
+
+ static inline bool atomic_inc_below(atomic_t *v, int u)
diff --git a/debian/patches/debian/userns-avoid-abi-change-for-cve-2017-6874-fix.patch b/debian/patches/debian/userns-avoid-abi-change-for-cve-2017-6874-fix.patch
new file mode 100644
index 0000000..58a6a58
--- /dev/null
+++ b/debian/patches/debian/userns-avoid-abi-change-for-cve-2017-6874-fix.patch
@@ -0,0 +1,23 @@
+From: Ben Hutchings <ben at decadent.org.uk>
+Date: Tue, 14 Mar 2017 21:35:33 +0000
+Subject: userns: Avoid ABI change for CVE-2017-6874 fix
+
+The type of ucounts::count changed from atomic_t to int. But they're
+the same size, and it's only accessed within kernel/ucount.c, so hide
+the change from genksyms.
+
+---
+--- a/include/linux/user_namespace.h
++++ b/include/linux/user_namespace.h
+@@ -65,7 +65,11 @@ struct ucounts {
+ struct hlist_node node;
+ struct user_namespace *ns;
+ kuid_t uid;
++#ifdef __GENKSYMS__
++ atomic_t count;
++#else
+ int count;
++#endif
+ atomic_t ucount[UCOUNT_COUNTS];
+ };
+
diff --git a/debian/patches/series b/debian/patches/series
index 7156e7f..e2f635a 100644
--- a/debian/patches/series
+++ b/debian/patches/series
@@ -116,6 +116,7 @@ bugfix/x86/kvm-fix-page-struct-leak-in-handle_vmon.patch
debian/time-mark-timer_stats-as-broken.patch
bugfix/all/sctp-deny-peeloff-operation-on-asocs-with-threads-sl.patch
bugfix/all/tty-n_hdlc-get-rid-of-racy-n_hdlc.patch
+bugfix/all/ucount-remove-the-atomicity-from-ucount-count.patch
# Fix exported symbol versions
bugfix/ia64/revert-ia64-move-exports-to-definitions.patch
@@ -129,6 +130,7 @@ bugfix/all/module-disable-matching-missing-version-crc.patch
# ABI maintenance
debian/net-avoid-abi-change-for-min_header_len.patch
+debian/userns-avoid-abi-change-for-cve-2017-6874-fix.patch
# Tools bug fixes
bugfix/all/usbip-document-tcp-wrappers.patch
--
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