From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Commit 34dbbcdbf63360661ff7bda6c5f52f99ac515f92 upstream. A lot of seqfile users seem to be using things like %pK that uses the credentials of the current process, but that is actually completely wrong for filesystem interfaces. The unix semantics for permission checking files is to check permissions at _open_ time, not at read or write time, and that is not just a small detail: passing off stdin/stdout/stderr to a suid application and making the actual IO happen in privileged context is a classic exploit technique. So if we want to be able to look at permissions at read time, we need to use the file open credentials, not the current ones. Normal file accesses can just use "f_cred" (or any of the helper functions that do that, like file_ns_capable()), but the seqfile interfaces do not have any such options. It turns out that seq_file _does_ save away the user_ns information of the file, though. Since user_ns is just part of the full credential information, replace that special case with saving off the cred pointer instead, and suddenly seq_file has all the permission information it needs. [sumits: this is used in Ubuntu as a fix for CVE-2015-8944] Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Sumit Semwal <sumit.semwal@xxxxxxxxxx> --- fs/seq_file.c | 7 ++++--- include/linux/seq_file.h | 13 ++++--------- 2 files changed, 8 insertions(+), 12 deletions(-) diff --git a/fs/seq_file.c b/fs/seq_file.c index d672e2fec459..6dc4296eed62 100644 --- a/fs/seq_file.c +++ b/fs/seq_file.c @@ -72,9 +72,10 @@ int seq_open(struct file *file, const struct seq_operations *op) mutex_init(&p->lock); p->op = op; -#ifdef CONFIG_USER_NS - p->user_ns = file->f_cred->user_ns; -#endif + + // No refcounting: the lifetime of 'p' is constrained + // to the lifetime of the file. + p->file = file; /* * Wrappers around seq_open(e.g. swaps_open) need to be diff --git a/include/linux/seq_file.h b/include/linux/seq_file.h index dde00defbaa5..f3d45dd42695 100644 --- a/include/linux/seq_file.h +++ b/include/linux/seq_file.h @@ -7,13 +7,10 @@ #include <linux/mutex.h> #include <linux/cpumask.h> #include <linux/nodemask.h> +#include <linux/fs.h> +#include <linux/cred.h> struct seq_operations; -struct file; -struct path; -struct inode; -struct dentry; -struct user_namespace; struct seq_file { char *buf; @@ -27,9 +24,7 @@ struct seq_file { struct mutex lock; const struct seq_operations *op; int poll_event; -#ifdef CONFIG_USER_NS - struct user_namespace *user_ns; -#endif + const struct file *file; void *private; }; @@ -147,7 +142,7 @@ int seq_release_private(struct inode *, struct file *); static inline struct user_namespace *seq_user_ns(struct seq_file *seq) { #ifdef CONFIG_USER_NS - return seq->user_ns; + return seq->file->f_cred->user_ns; #else extern struct user_namespace init_user_ns; return &init_user_ns; -- 2.7.4