[PATCH 4.19 62/88] uaccess: Add non-pagefault user-space write function

Greg Kroah-Hartman
2020-09-08T18:32:04Z

From: Daniel Borkmann <daniel@iogearbox.net>

[ Upstream commit 1d1585ca0f48fe7ed95c3571f3e4a82b2b5045dc ]

Commit 3d7081822f7f ("uaccess: Add non-pagefault user-space read functions")
missed to add probe write function, therefore factor out a probe_write_common()
helper with most logic of probe_kernel_write() except setting KERNEL_DS, and
add a new probe_user_write() helper so it can be used from BPF side.

Again, on some archs, the user address space and kernel address space can
co-exist and be overlapping, so in such case, setting KERNEL_DS would mean
that the given address is treated as being in kernel address space.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Link: https://lore.kernel.org/bpf/9df2542e68141bfa3addde631441ee45503856a8.1572649915.git.daniel@iogearbox.net
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
include/linux/uaccess.h | 12 +++++++++++
mm/maccess.c | 45 +++++++++++++++++++++++++++++++++++++----
2 files changed, 53 insertions(+), 4 deletions(-)

diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index db88f36540e9e..db9b0dd0a7a3b 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -265,6 +265,18 @@ extern long probe_user_read(void *dst, const void __user *src, size_t size);
extern long notrace probe_kernel_write(void *dst, const void *src, size_t size);
extern long notrace __probe_kernel_write(void *dst, const void *src, size_t size);

+/*
+ * probe_user_write(): safely attempt to write to a location in user space
+ * @dst: address to write to
+ * @src: pointer to the data that shall be written
+ * @size: size of the data chunk
+ *
+ * Safely write to address @dst from the buffer at @src. If a kernel fault
+ * happens, handle that and return -EFAULT.
+ */
+extern long notrace probe_user_write(void __user *dst, const void *src, size_t size);
+extern long notrace __probe_user_write(void __user *dst, const void *src, size_t size);
+
extern long strncpy_from_unsafe(char *dst, const void *unsafe_addr, long count);
extern long strncpy_from_unsafe_user(char *dst, const void __user *unsafe_addr,
long count);
diff --git a/mm/maccess.c b/mm/maccess.c
index 80d70cb5cc0bd..6e41ba452e5e9 100644
--- a/mm/maccess.c
+++ b/mm/maccess.c
@@ -17,6 +17,18 @@ probe_read_common(void *dst, const void __user *src, size_t size)
return ret ? -EFAULT : 0;
}

+static __always_inline long
+probe_write_common(void __user *dst, const void *src, size_t size)
+{
+ long ret;
+
+ pagefault_disable();
+ ret = __copy_to_user_inatomic(dst, src, size);
+ pagefault_enable();
+
+ return ret ? -EFAULT : 0;
+}
+
/**
* probe_kernel_read(): safely attempt to read from a kernel-space location
* @dst: pointer to the buffer that shall take the data
@@ -84,6 +96,7 @@ EXPORT_SYMBOL_GPL(probe_user_read);
* Safely write to address @dst from the buffer at @src. If a kernel fault
* happens, handle that and return -EFAULT.
*/
+
long __weak probe_kernel_write(void *dst, const void *src, size_t size)
__attribute__((alias("__probe_kernel_write")));

@@ -93,15 +106,39 @@ long __probe_kernel_write(void *dst, const void *src, size_t size)
mm_segment_t old_fs = get_fs();

set_fs(KERNEL_DS);
- pagefault_disable();
- ret = __copy_to_user_inatomic((__force void __user *)dst, src, size);
- pagefault_enable();
+ ret = probe_write_common((__force void __user *)dst, src, size);
set_fs(old_fs);

- return ret ? -EFAULT : 0;
+ return ret;
}
EXPORT_SYMBOL_GPL(probe_kernel_write);

+/**
+ * probe_user_write(): safely attempt to write to a user-space location
+ * @dst: address to write to
+ * @src: pointer to the data that shall be written
+ * @size: size of the data chunk
+ *
+ * Safely write to address @dst from the buffer at @src. If a kernel fault
+ * happens, handle that and return -EFAULT.
+ */
+
+long __weak probe_user_write(void __user *dst, const void *src, size_t size)
+ __attribute__((alias("__probe_user_write")));
+
+long __probe_user_write(void __user *dst, const void *src, size_t size)
+{
+ long ret = -EFAULT;
+ mm_segment_t old_fs = get_fs();
+
+ set_fs(USER_DS);
+ if (access_ok(VERIFY_WRITE, dst, size))
+ ret = probe_write_common(dst, src, size);
+ set_fs(old_fs);
+
+ return ret;
+}
+EXPORT_SYMBOL_GPL(probe_user_write);

/**
* strncpy_from_unsafe: - Copy a NUL terminated string from unsafe address.
--
2.25.1