copy_mount_string: Limit string length to PATH_MAX

On ppc64le, When a string with PAGE_SIZE - 1 (i.e. 64k-1) length is
passed as a "filesystem type" argument to the mount(2) syscall,
copy_mount_string() ends up allocating 64k (the PAGE_SIZE on ppc64le)
worth of space for holding the string in kernel's address space.

Later, in set_precision() (invoked by get_fs_type() ->
__request_module() -> vsnprintf()), we end up assigning
strlen(fs-type-string) i.e. 65535 as the
value to 'struct printf_spec'->precision member. This field has a width
of 16 bits and it is a 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 limiting the length of the string passed by
copy_mount_string() to strndup_user() to PATH_MAX.

Signed-off-by: Chandan Rajendra <chandan@linux.ibm.com>
Reported-by: Abdul Haleem <abdhalee@linux.ibm.com>
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/namespace.c b/fs/namespace.c
index a677b59efd74e25f52fd3db3e610c6c5193fb171..c373c769e0ce6ebda79de16873373abc7d6be482 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2746,7 +2746,7 @@ void *copy_mount_options(const void __user * data)
 
 char *copy_mount_string(const void __user *data)
 {
-       return data ? strndup_user(data, PAGE_SIZE) : NULL;
+       return data ? strndup_user(data, PATH_MAX) : NULL;
 }
 
 /*