3.18-stable review patch. If anyone has any objections, please let me know.
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From: Linus Torvalds torvalds@linux-foundation.org
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.
Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Cc: Jann Horn jannh@google.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- fs/seq_file.c | 7 ++++--- include/linux/seq_file.h | 13 ++++--------- 2 files changed, 8 insertions(+), 12 deletions(-)
--- a/fs/seq_file.c +++ b/fs/seq_file.c @@ -69,9 +69,10 @@ int seq_open(struct file *file, const st memset(p, 0, sizeof(*p)); 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 --- 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; };
@@ -151,7 +146,7 @@ int seq_put_decimal_ll(struct seq_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;