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authorAleksa Sarai <cyphar@cyphar.com>2019-10-01 11:10:52 +1000
committerChristian Brauner <christian.brauner@ubuntu.com>2019-10-01 15:45:03 +0200
commitf5a1a536fa14895ccff4e94e6a5af90901ce86aa (patch)
tree04c131cf6206c2dc929dd2ccf6341adfa1a2e814 /lib/usercopy.c
parent54ecb8f7028c5eb3d740bb82b0f1d90f2df63c5c (diff)
lib: introduce copy_struct_from_user() helper
A common pattern for syscall extensions is increasing the size of a struct passed from userspace, such that the zero-value of the new fields result in the old kernel behaviour (allowing for a mix of userspace and kernel vintages to operate on one another in most cases). While this interface exists for communication in both directions, only one interface is straightforward to have reasonable semantics for (userspace passing a struct to the kernel). For kernel returns to userspace, what the correct semantics are (whether there should be an error if userspace is unaware of a new extension) is very syscall-dependent and thus probably cannot be unified between syscalls (a good example of this problem is [1]). Previously there was no common lib/ function that implemented the necessary extension-checking semantics (and different syscalls implemented them slightly differently or incompletely[2]). Future patches replace common uses of this pattern to make use of copy_struct_from_user(). Some in-kernel selftests that insure that the handling of alignment and various byte patterns are all handled identically to memchr_inv() usage. [1]: commit 1251201c0d34 ("sched/core: Fix uclamp ABI bug, clean up and robustify sched_read_attr() ABI logic and code") [2]: For instance {sched_setattr,perf_event_open,clone3}(2) all do do similar checks to copy_struct_from_user() while rt_sigprocmask(2) always rejects differently-sized struct arguments. Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> Reviewed-by: Kees Cook <keescook@chromium.org> Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com> Link: https://lore.kernel.org/r/20191001011055.19283-2-cyphar@cyphar.com Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Diffstat (limited to 'lib/usercopy.c')
-rw-r--r--lib/usercopy.c55
1 files changed, 55 insertions, 0 deletions
diff --git a/lib/usercopy.c b/lib/usercopy.c
index c2bfbcaeb3dc..cbb4d9ec00f2 100644
--- a/lib/usercopy.c
+++ b/lib/usercopy.c
@@ -1,5 +1,6 @@
// SPDX-License-Identifier: GPL-2.0
#include <linux/uaccess.h>
+#include <linux/bitops.h>
/* out-of-line parts */
@@ -31,3 +32,57 @@ unsigned long _copy_to_user(void __user *to, const void *from, unsigned long n)
}
EXPORT_SYMBOL(_copy_to_user);
#endif
+
+/**
+ * check_zeroed_user: check if a userspace buffer only contains zero bytes
+ * @from: Source address, in userspace.
+ * @size: Size of buffer.
+ *
+ * This is effectively shorthand for "memchr_inv(from, 0, size) == NULL" for
+ * userspace addresses (and is more efficient because we don't care where the
+ * first non-zero byte is).
+ *
+ * Returns:
+ * * 0: There were non-zero bytes present in the buffer.
+ * * 1: The buffer was full of zero bytes.
+ * * -EFAULT: access to userspace failed.
+ */
+int check_zeroed_user(const void __user *from, size_t size)
+{
+ unsigned long val;
+ uintptr_t align = (uintptr_t) from % sizeof(unsigned long);
+
+ if (unlikely(size == 0))
+ return 1;
+
+ from -= align;
+ size += align;
+
+ if (!user_access_begin(from, size))
+ return -EFAULT;
+
+ unsafe_get_user(val, (unsigned long __user *) from, err_fault);
+ if (align)
+ val &= ~aligned_byte_mask(align);
+
+ while (size > sizeof(unsigned long)) {
+ if (unlikely(val))
+ goto done;
+
+ from += sizeof(unsigned long);
+ size -= sizeof(unsigned long);
+
+ unsafe_get_user(val, (unsigned long __user *) from, err_fault);
+ }
+
+ if (size < sizeof(unsigned long))
+ val &= aligned_byte_mask(size);
+
+done:
+ user_access_end();
+ return (val == 0);
+err_fault:
+ user_access_end();
+ return -EFAULT;
+}
+EXPORT_SYMBOL(check_zeroed_user);