[PATCH V2] get_fs_type: Validate fs type string argument

[Chandan Rajendra <chandan@linux.vnet.ibm.com>]

On ppc64, When a non-nul terminated string is passed as an argument to
the mount(2) syscall, copy_mount_string() ends up allocating 64k (the
PAGE_SIZE on ppc64) worth of space for holding the string in kernel's
address space.

Later, in set_precision() (invoked indirectly by get_fs_type()), we end
up assigning 65535 as the value to 'struct printf_spec'->precision
member. This field has a width of 16 bits and also it is signed data
type. Hence an invalid value ends up getting assigned. This causes the
"WARN_ONCE(spec->precision != prec, "precision %d too large", prec)"
statement inside set_precision() to be executed.

This commit fixes the bug by validating the length of the "filesystem
type" argument passed to get_fs_type() function.

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Reported-by: Abdul Haleem <abdhalee@linux.vnet.ibm.com>
Suggested-by: Joe Perches <joe@perches.com>
---
Changelog:
v1->v2: Fix commit message.

 fs/filesystems.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/filesystems.c b/fs/filesystems.c
index a920ad2..8e309d3 100644
--- a/fs/filesystems.c
+++ b/fs/filesystems.c
@@ -275,6 +275,9 @@ struct file_system_type *get_fs_type(const char *name)
 	const char *dot = strchr(name, '.');
 	int len = dot ? dot - name : strlen(name);
 
+       if (len >= PATH_MAX)
+               return NULL;
+
 	fs = __get_fs_type(name, len);
 	if (!fs && (request_module("fs-%.*s", len, name) == 0)) {
 		fs = __get_fs_type(name, len);
-- 
2.9.4