4.17-stable review patch. If anyone has any objections, please let me know.
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From: Linus Torvalds torvalds@linux-foundation.org
commit 0fa3ecd87848c9c93c2c828ef4c3a8ca36ce46c7 upstream.
sgid directories have special semantics, making newly created files in the directory belong to the group of the directory, and newly created subdirectories will also become sgid. This is historically used for group-shared directories.
But group directories writable by non-group members should not imply that such non-group members can magically join the group, so make sure to clear the sgid bit on non-directories for non-members (but remember that sgid without group execute means "mandatory locking", just to confuse things even more).
Reported-by: Jann Horn jannh@google.com Cc: Andy Lutomirski luto@kernel.org Cc: Al Viro viro@zeniv.linux.org.uk Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- fs/inode.c | 6 ++++++ 1 file changed, 6 insertions(+)
--- a/fs/inode.c +++ b/fs/inode.c @@ -2013,8 +2013,14 @@ void inode_init_owner(struct inode *inod inode->i_uid = current_fsuid(); if (dir && dir->i_mode & S_ISGID) { inode->i_gid = dir->i_gid; + + /* Directories are special, and always inherit S_ISGID */ if (S_ISDIR(mode)) mode |= S_ISGID; + else if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP) && + !in_group_p(inode->i_gid) && + !capable_wrt_inode_uidgid(dir, CAP_FSETID)) + mode &= ~S_ISGID; } else inode->i_gid = current_fsgid(); inode->i_mode = mode;