1 files changed, 56 insertions, 8 deletions

diff --git a/fs/coredump.c b/fs/coredump.c
index 4c5866b948e7..7eb6181184ea 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -32,6 +32,9 @@
 #include <linux/pipe_fs_i.h>
 #include <linux/oom.h>
 #include <linux/compat.h>
+#include <linux/sched.h>
+#include <linux/fs.h>
+#include <linux/path.h>
 
 #include <asm/uaccess.h>
 #include <asm/mmu_context.h>
@@ -506,10 +509,10 @@ void do_coredump(const siginfo_t *siginfo)
 	const struct cred *old_cred;
 	struct cred *cred;
 	int retval = 0;
-	int flag = 0;
 	int ispipe;
 	struct files_struct *displaced;
-	bool need_nonrelative = false;
+	/* require nonrelative corefile path and be extra careful */
+	bool need_suid_safe = false;
 	bool core_dumped = false;
 	static atomic_t core_dump_count = ATOMIC_INIT(0);
 	struct coredump_params cprm = {
@@ -543,9 +546,8 @@ void do_coredump(const siginfo_t *siginfo)
 	 */
 	if (__get_dumpable(cprm.mm_flags) == SUID_DUMP_ROOT) {
 		/* Setuid core dump mode */
-		flag = O_EXCL;		/* Stop rewrite attacks */
 		cred->fsuid = GLOBAL_ROOT_UID;	/* Dump root private */
-		need_nonrelative = true;
+		need_suid_safe = true;
 	}
 
 	retval = coredump_wait(siginfo->si_signo, &core_state);
@@ -622,11 +624,13 @@ void do_coredump(const siginfo_t *siginfo)
 		}
 	} else {
 		struct inode *inode;
+		int open_flags = O_CREAT | O_RDWR | O_NOFOLLOW |
+				 O_LARGEFILE | O_EXCL;
 
 		if (cprm.limit < binfmt->min_coredump)
 			goto fail_unlock;
 
-		if (need_nonrelative && cn.corename[0] != '/') {
+		if (need_suid_safe && cn.corename[0] != '/') {
 			printk(KERN_WARNING "Pid %d(%s) can only dump core "\
 				"to fully qualified path!\n",
 				task_tgid_vnr(current), current->comm);
@@ -634,9 +638,53 @@ void do_coredump(const siginfo_t *siginfo)
 			goto fail_unlock;
 		}
 
-		cprm.file = filp_open(cn.corename,
-				 O_CREAT | 2 | O_NOFOLLOW | O_LARGEFILE | flag,
-				 0600);
+		/*
+		 * Unlink the file if it exists unless this is a SUID
+		 * binary - in that case, we're running around with root
+		 * privs and don't want to unlink another user's coredump.
+		 */
+		if (!need_suid_safe) {
+			mm_segment_t old_fs;
+
+			old_fs = get_fs();
+			set_fs(KERNEL_DS);
+			/*
+			 * If it doesn't exist, that's fine. If there's some
+			 * other problem, we'll catch it at the filp_open().
+			 */
+			(void) sys_unlink((const char __user *)cn.corename);
+			set_fs(old_fs);
+		}
+
+		/*
+		 * There is a race between unlinking and creating the
+		 * file, but if that causes an EEXIST here, that's
+		 * fine - another process raced with us while creating
+		 * the corefile, and the other process won. To userspace,
+		 * what matters is that at least one of the two processes
+		 * writes its coredump successfully, not which one.
+		 */
+		if (need_suid_safe) {
+			/*
+			 * Using user namespaces, normal user tasks can change
+			 * their current->fs->root to point to arbitrary
+			 * directories. Since the intention of the "only dump
+			 * with a fully qualified path" rule is to control where
+			 * coredumps may be placed using root privileges,
+			 * current->fs->root must not be used. Instead, use the
+			 * root directory of init_task.
+			 */
+			struct path root;
+
+			task_lock(&init_task);
+			get_fs_root(init_task.fs, &root);
+			task_unlock(&init_task);
+			cprm.file = file_open_root(root.dentry, root.mnt,
+				cn.corename, open_flags, 0600);
+			path_put(&root);
+		} else {
+			cprm.file = filp_open(cn.corename, open_flags, 0600);
+		}
 		if (IS_ERR(cprm.file))
 			goto fail_unlock;