Message-Id: <20180625163410.216168-1-jannh@google.com>
Date:   Mon, 25 Jun 2018 18:34:10 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        linux-kernel@...r.kernel.org,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Al Viro <viro@...iv.linux.org.uk>, jannh@...gle.com
Cc:     linux-alpha@...r.kernel.org, sparclinux@...r.kernel.org,
        security@...nel.org, Christoph Hellwig <hch@...radead.org>,
        Serge Hallyn <serge@...lyn.com>
Subject: [PATCH] sys: don't hold uts_sem while accessing userspace memory

Holding uts_sem as a writer while accessing userspace memory allows a
namespace admin to stall all processes that attempt to take uts_sem.
Instead, move data through stack buffers and don't access userspace memory
while uts_sem is held.

Cc: stable@...r.kernel.org
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Jann Horn <jannh@...gle.com>
---
 arch/alpha/kernel/osf_sys.c      | 51 ++++++++---------
 arch/sparc/kernel/sys_sparc_32.c | 22 +++++---
 arch/sparc/kernel/sys_sparc_64.c | 20 ++++---
 kernel/sys.c                     | 95 +++++++++++++++-----------------
 kernel/utsname_sysctl.c          | 41 ++++++++------
 5 files changed, 119 insertions(+), 110 deletions(-)

diff --git a/arch/alpha/kernel/osf_sys.c b/arch/alpha/kernel/osf_sys.c
index 6e921754c8fc..2a5c768df335 100644
--- a/arch/alpha/kernel/osf_sys.c
+++ b/arch/alpha/kernel/osf_sys.c
@@ -530,24 +530,19 @@ SYSCALL_DEFINE4(osf_mount, unsigned long, typenr, const char __user *, path,
 SYSCALL_DEFINE1(osf_utsname, char __user *, name)
 {
 	int error;
+	char tmp[5 * 32];
 
 	down_read(&uts_sem);
-	error = -EFAULT;
-	if (copy_to_user(name + 0, utsname()->sysname, 32))
-		goto out;
-	if (copy_to_user(name + 32, utsname()->nodename, 32))
-		goto out;
-	if (copy_to_user(name + 64, utsname()->release, 32))
-		goto out;
-	if (copy_to_user(name + 96, utsname()->version, 32))
-		goto out;
-	if (copy_to_user(name + 128, utsname()->machine, 32))
-		goto out;
+	memcpy(tmp + 0 * 32, utsname()->sysname, 32);
+	memcpy(tmp + 1 * 32, utsname()->nodename, 32);
+	memcpy(tmp + 2 * 32, utsname()->release, 32);
+	memcpy(tmp + 3 * 32, utsname()->version, 32);
+	memcpy(tmp + 4 * 32, utsname()->machine, 32);
+	up_read(&uts_sem);
 
-	error = 0;
- out:
-	up_read(&uts_sem);	
-	return error;
+	if (copy_to_user(name, tmp, sizeof(tmp)))
+		return -EFAULT;
+	return 0;
 }
 
 SYSCALL_DEFINE0(getpagesize)
@@ -567,18 +562,21 @@ SYSCALL_DEFINE2(osf_getdomainname, char __user *, name, int, namelen)
 {
 	int len, err = 0;
 	char *kname;
+	char tmp[32];
 
-	if (namelen > 32)
+	if (namelen < 0 || namelen > 32)
 		namelen = 32;
 
 	down_read(&uts_sem);
 	kname = utsname()->domainname;
 	len = strnlen(kname, namelen);
-	if (copy_to_user(name, kname, min(len + 1, namelen)))
-		err = -EFAULT;
+	len = min(len + 1, namelen);
+	memcpy(tmp, kname, len);
 	up_read(&uts_sem);
 
-	return err;
+	if (copy_to_user(name, tmp, len))
+		return -EFAULT;
+	return 0;
 }
 
 /*
@@ -739,13 +737,14 @@ SYSCALL_DEFINE3(osf_sysinfo, int, command, char __user *, buf, long, count)
 	};
 	unsigned long offset;
 	const char *res;
-	long len, err = -EINVAL;
+	long len;
+	char tmp[__NEW_UTS_LEN + 1];
 
 	offset = command-1;
 	if (offset >= ARRAY_SIZE(sysinfo_table)) {
 		/* Digital UNIX has a few unpublished interfaces here */
 		printk("sysinfo(%d)", command);
-		goto out;
+		return -EINVAL;
 	}
 
 	down_read(&uts_sem);
@@ -753,13 +752,11 @@ SYSCALL_DEFINE3(osf_sysinfo, int, command, char __user *, buf, long, count)
 	len = strlen(res)+1;
 	if ((unsigned long)len > (unsigned long)count)
 		len = count;
-	if (copy_to_user(buf, res, len))
-		err = -EFAULT;
-	else
-		err = 0;
+	memcpy(tmp, res, len);
 	up_read(&uts_sem);
- out:
-	return err;
+	if (copy_to_user(buf, tmp, len))
+		return -EFAULT;
+	return 0;
 }
 
 SYSCALL_DEFINE5(osf_getsysinfo, unsigned long, op, void __user *, buffer,
diff --git a/arch/sparc/kernel/sys_sparc_32.c b/arch/sparc/kernel/sys_sparc_32.c
index 7f3d9c59719a..452e4d080855 100644
--- a/arch/sparc/kernel/sys_sparc_32.c
+++ b/arch/sparc/kernel/sys_sparc_32.c
@@ -197,23 +197,27 @@ SYSCALL_DEFINE5(rt_sigaction, int, sig,
 
 SYSCALL_DEFINE2(getdomainname, char __user *, name, int, len)
 {
- 	int nlen, err;
- 	
+	int nlen, err;
+	char tmp[__NEW_UTS_LEN + 1];
+
 	if (len < 0)
 		return -EINVAL;
 
- 	down_read(&uts_sem);
- 	
+	down_read(&uts_sem);
+
 	nlen = strlen(utsname()->domainname) + 1;
 	err = -EINVAL;
 	if (nlen > len)
-		goto out;
+		goto out_unlock;
+	memcpy(tmp, utsname()->domainname, nlen);
 
-	err = -EFAULT;
-	if (!copy_to_user(name, utsname()->domainname, nlen))
-		err = 0;
+	up_read(&uts_sem);
 
-out:
+	if (copy_to_user(name, tmp, nlen))
+		return -EFAULT;
+	return 0;
+
+out_unlock:
 	up_read(&uts_sem);
 	return err;
 }
diff --git a/arch/sparc/kernel/sys_sparc_64.c b/arch/sparc/kernel/sys_sparc_64.c
index 63baa8aa9414..274ed0b9b3e0 100644
--- a/arch/sparc/kernel/sys_sparc_64.c
+++ b/arch/sparc/kernel/sys_sparc_64.c
@@ -519,23 +519,27 @@ asmlinkage void sparc_breakpoint(struct pt_regs *regs)
 
 SYSCALL_DEFINE2(getdomainname, char __user *, name, int, len)
 {
-        int nlen, err;
+	int nlen, err;
+	char tmp[__NEW_UTS_LEN + 1];
 
 	if (len < 0)
 		return -EINVAL;
 
- 	down_read(&uts_sem);
- 	
+	down_read(&uts_sem);
+
 	nlen = strlen(utsname()->domainname) + 1;
 	err = -EINVAL;
 	if (nlen > len)
-		goto out;
+		goto out_unlock;
+	memcpy(tmp, utsname()->domainname, nlen);
+
+	up_read(&uts_sem);
 
-	err = -EFAULT;
-	if (!copy_to_user(name, utsname()->domainname, nlen))
-		err = 0;
+	if (copy_to_user(name, tmp, nlen))
+		return -EFAULT;
+	return 0;
 
-out:
+out_unlock:
 	up_read(&uts_sem);
 	return err;
 }
diff --git a/kernel/sys.c b/kernel/sys.c
index 38509dc1f77b..69b9a37ecf0d 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1237,18 +1237,19 @@ static int override_release(char __user *release, size_t len)
 
 SYSCALL_DEFINE1(newuname, struct new_utsname __user *, name)
 {
-	int errno = 0;
+	struct new_utsname tmp;
 
 	down_read(&uts_sem);
-	if (copy_to_user(name, utsname(), sizeof *name))
-		errno = -EFAULT;
+	memcpy(&tmp, utsname(), sizeof(tmp));
 	up_read(&uts_sem);
+	if (copy_to_user(name, &tmp, sizeof(tmp)))
+		return -EFAULT;
 
-	if (!errno && override_release(name->release, sizeof(name->release)))
-		errno = -EFAULT;
-	if (!errno && override_architecture(name))
-		errno = -EFAULT;
-	return errno;
+	if (override_release(name->release, sizeof(name->release)))
+		return -EFAULT;
+	if (override_architecture(name))
+		return -EFAULT;
+	return 0;
 }
 
 #ifdef __ARCH_WANT_SYS_OLD_UNAME
@@ -1257,55 +1258,46 @@ SYSCALL_DEFINE1(newuname, struct new_utsname __user *, name)
  */
 SYSCALL_DEFINE1(uname, struct old_utsname __user *, name)
 {
-	int error = 0;
+	struct old_utsname tmp;
 
 	if (!name)
 		return -EFAULT;
 
 	down_read(&uts_sem);
-	if (copy_to_user(name, utsname(), sizeof(*name)))
-		error = -EFAULT;
+	memcpy(&tmp, utsname(), sizeof(tmp));
 	up_read(&uts_sem);
+	if (copy_to_user(name, &tmp, sizeof(tmp)))
+		return -EFAULT;
 
-	if (!error && override_release(name->release, sizeof(name->release)))
-		error = -EFAULT;
-	if (!error && override_architecture(name))
-		error = -EFAULT;
-	return error;
+	if (override_release(name->release, sizeof(name->release)))
+		return -EFAULT;
+	if (override_architecture(name))
+		return -EFAULT;
+	return 0;
 }
 
 SYSCALL_DEFINE1(olduname, struct oldold_utsname __user *, name)
 {
-	int error;
+	struct oldold_utsname tmp = {};
 
 	if (!name)
 		return -EFAULT;
-	if (!access_ok(VERIFY_WRITE, name, sizeof(struct oldold_utsname)))
-		return -EFAULT;
 
 	down_read(&uts_sem);
-	error = __copy_to_user(&name->sysname, &utsname()->sysname,
-			       __OLD_UTS_LEN);
-	error |= __put_user(0, name->sysname + __OLD_UTS_LEN);
-	error |= __copy_to_user(&name->nodename, &utsname()->nodename,
-				__OLD_UTS_LEN);
-	error |= __put_user(0, name->nodename + __OLD_UTS_LEN);
-	error |= __copy_to_user(&name->release, &utsname()->release,
-				__OLD_UTS_LEN);
-	error |= __put_user(0, name->release + __OLD_UTS_LEN);
-	error |= __copy_to_user(&name->version, &utsname()->version,
-				__OLD_UTS_LEN);
-	error |= __put_user(0, name->version + __OLD_UTS_LEN);
-	error |= __copy_to_user(&name->machine, &utsname()->machine,
-				__OLD_UTS_LEN);
-	error |= __put_user(0, name->machine + __OLD_UTS_LEN);
+	memcpy(&tmp.sysname, &utsname()->sysname, __OLD_UTS_LEN);
+	memcpy(&tmp.nodename, &utsname()->nodename, __OLD_UTS_LEN);
+	memcpy(&tmp.release, &utsname()->release, __OLD_UTS_LEN);
+	memcpy(&tmp.version, &utsname()->version, __OLD_UTS_LEN);
+	memcpy(&tmp.machine, &utsname()->machine, __OLD_UTS_LEN);
 	up_read(&uts_sem);
+	if (copy_to_user(name, &tmp, sizeof(tmp)))
+		return -EFAULT;
 
-	if (!error && override_architecture(name))
-		error = -EFAULT;
-	if (!error && override_release(name->release, sizeof(name->release)))
-		error = -EFAULT;
-	return error ? -EFAULT : 0;
+	if (override_architecture(name))
+		return -EFAULT;
+	if (override_release(name->release, sizeof(name->release)))
+		return -EFAULT;
+	return 0;
 }
 #endif
 
@@ -1319,17 +1311,18 @@ SYSCALL_DEFINE2(sethostname, char __user *, name, int, len)
 
 	if (len < 0 || len > __NEW_UTS_LEN)
 		return -EINVAL;
-	down_write(&uts_sem);
 	errno = -EFAULT;
 	if (!copy_from_user(tmp, name, len)) {
-		struct new_utsname *u = utsname();
+		struct new_utsname *u;
 
+		down_write(&uts_sem);
+		u = utsname();
 		memcpy(u->nodename, tmp, len);
 		memset(u->nodename + len, 0, sizeof(u->nodename) - len);
 		errno = 0;
 		uts_proc_notify(UTS_PROC_HOSTNAME);
+		up_write(&uts_sem);
 	}
-	up_write(&uts_sem);
 	return errno;
 }
 
@@ -1337,8 +1330,9 @@ SYSCALL_DEFINE2(sethostname, char __user *, name, int, len)
 
 SYSCALL_DEFINE2(gethostname, char __user *, name, int, len)
 {
-	int i, errno;
+	int i;
 	struct new_utsname *u;
+	char tmp[__NEW_UTS_LEN + 1];
 
 	if (len < 0)
 		return -EINVAL;
@@ -1347,11 +1341,11 @@ SYSCALL_DEFINE2(gethostname, char __user *, name, int, len)
 	i = 1 + strlen(u->nodename);
 	if (i > len)
 		i = len;
-	errno = 0;
-	if (copy_to_user(name, u->nodename, i))
-		errno = -EFAULT;
+	memcpy(tmp, u->nodename, i);
 	up_read(&uts_sem);
-	return errno;
+	if (copy_to_user(name, tmp, i))
+		return -EFAULT;
+	return 0;
 }
 
 #endif
@@ -1370,17 +1364,18 @@ SYSCALL_DEFINE2(setdomainname, char __user *, name, int, len)
 	if (len < 0 || len > __NEW_UTS_LEN)
 		return -EINVAL;
 
-	down_write(&uts_sem);
 	errno = -EFAULT;
 	if (!copy_from_user(tmp, name, len)) {
-		struct new_utsname *u = utsname();
+		struct new_utsname *u;
 
+		down_write(&uts_sem);
+		u = utsname();
 		memcpy(u->domainname, tmp, len);
 		memset(u->domainname + len, 0, sizeof(u->domainname) - len);
 		errno = 0;
 		uts_proc_notify(UTS_PROC_DOMAINNAME);
+		up_write(&uts_sem);
 	}
-	up_write(&uts_sem);
 	return errno;
 }
 
diff --git a/kernel/utsname_sysctl.c b/kernel/utsname_sysctl.c
index 233cd8fc6910..258033d62cb3 100644
--- a/kernel/utsname_sysctl.c
+++ b/kernel/utsname_sysctl.c
@@ -18,7 +18,7 @@
 
 #ifdef CONFIG_PROC_SYSCTL
 
-static void *get_uts(struct ctl_table *table, int write)
+static void *get_uts(struct ctl_table *table)
 {
 	char *which = table->data;
 	struct uts_namespace *uts_ns;
@@ -26,21 +26,9 @@ static void *get_uts(struct ctl_table *table, int write)
 	uts_ns = current->nsproxy->uts_ns;
 	which = (which - (char *)&init_uts_ns) + (char *)uts_ns;
 
-	if (!write)
-		down_read(&uts_sem);
-	else
-		down_write(&uts_sem);
 	return which;
 }
 
-static void put_uts(struct ctl_table *table, int write, void *which)
-{
-	if (!write)
-		up_read(&uts_sem);
-	else
-		up_write(&uts_sem);
-}
-
 /*
  *	Special case of dostring for the UTS structure. This has locks
  *	to observe. Should this be in kernel/sys.c ????
@@ -50,13 +38,34 @@ static int proc_do_uts_string(struct ctl_table *table, int write,
 {
 	struct ctl_table uts_table;
 	int r;
+	char tmp_data[__NEW_UTS_LEN + 1];
+
 	memcpy(&uts_table, table, sizeof(uts_table));
-	uts_table.data = get_uts(table, write);
+	uts_table.data = tmp_data;
+
+	/*
+	 * Buffer the value in tmp_data so that proc_dostring() can be called
+	 * without holding any locks.
+	 * We also need to read the original value in the write==1 case to
+	 * support partial writes.
+	 */
+	down_read(&uts_sem);
+	memcpy(tmp_data, get_uts(table), sizeof(tmp_data));
+	up_read(&uts_sem);
 	r = proc_dostring(&uts_table, write, buffer, lenp, ppos);
-	put_uts(table, write, uts_table.data);
 
-	if (write)
+	if (write) {
+		/*
+		 * Write back the new value.
+		 * Note that, since we dropped uts_sem, the result can
+		 * theoretically be incorrect if there are two parallel writes
+		 * at non-zero offsets to the same sysctl.
+		 */
+		down_write(&uts_sem);
+		memcpy(get_uts(table), tmp_data, sizeof(tmp_data));
+		up_write(&uts_sem);
 		proc_sys_poll_notify(table->poll);
+	}
 
 	return r;
 }
-- 
2.18.0.rc2.346.g013aa6912e-goog

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