fs/coredump.c: log if a core dump is aborted due to changed file permissions

For obvious security reasons, a core dump is aborted if the filesystem
cannot preserve ownership or permissions of the dump file.

This affects filesystems like e.g.  vfat, but also something like a 9pfs
share in a Qemu test setup, running as a regular user, depending on the
security model used.  In those cases, the result is an empty core file and
a confused user.

To hopefully save other people a lot of time figuring out the cause, this
patch adds a simple log message for those specific cases.

[akpm@linux-foundation.org: s/|%s/%s/ in printk text]

Link: https://lkml.kernel.org/r/20210701233151.102720-1-david.oberhollenzer@sigma-star.at
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/coredump.c b/fs/coredump.c
index 07afb5d..4b3c757 100644 (file)
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -782,10 +782,17 @@ void do_coredump(const kernel_siginfo_t *siginfo)
                 * filesystem.
                 */
                mnt_userns = file_mnt_user_ns(cprm.file);
-               if (!uid_eq(i_uid_into_mnt(mnt_userns, inode), current_fsuid()))
+               if (!uid_eq(i_uid_into_mnt(mnt_userns, inode),
+                           current_fsuid())) {
+                       pr_info_ratelimited("Core dump to %s aborted: cannot preserve file owner\n",
+                                           cn.corename);
                        goto close_fail;
-               if ((inode->i_mode & 0677) != 0600)
+               }
+               if ((inode->i_mode & 0677) != 0600) {
+                       pr_info_ratelimited("Core dump to %s aborted: cannot preserve file permissions\n",
+                                           cn.corename);
                        goto close_fail;
+               }
                if (!(cprm.file->f_mode & FMODE_CAN_WRITE))
                        goto close_fail;
                if (do_truncate(mnt_userns, cprm.file->f_path.dentry,