maccess: always use strict semantics for probe_kernel_read

Except for historical confusion in the kprobes/uprobes and bpf tracers,
which has been fixed now, there is no good reason to ever allow user
memory accesses from probe_kernel_read.  Switch probe_kernel_read to only
read from kernel memory.

[akpm@linux-foundation.org: update it for "mm, dump_page(): do not crash with invalid mapping pointer"]

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20200521152301.2587579-17-hch@lst.de
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 11 insertions, 39 deletions

diff --git a/mm/debug.c b/mm/debug.c
index f2ede2df585a..b5b1de8c71ac 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -120,9 +120,9 @@ void __dump_page(struct page *page, const char *reason)
 		 * mapping can be invalid pointer and we don't want to crash
 		 * accessing it, so probe everything depending on it carefully
 		 */
-		if (probe_kernel_read_strict(&host, &mapping->host,
-						sizeof(struct inode *)) ||
-		    probe_kernel_read_strict(&a_ops, &mapping->a_ops,
+		if (probe_kernel_read(&host, &mapping->host,
+					sizeof(struct inode *)) ||
+		    probe_kernel_read(&a_ops, &mapping->a_ops,
 				sizeof(struct address_space_operations *))) {
 			pr_warn("failed to read mapping->host or a_ops, mapping not a valid kernel address?\n");
 			goto out_mapping;
@@ -133,7 +133,7 @@ void __dump_page(struct page *page, const char *reason)
 			goto out_mapping;
 		}
 
-		if (probe_kernel_read_strict(&dentry_first,
+		if (probe_kernel_read(&dentry_first,
 			&host->i_dentry.first, sizeof(struct hlist_node *))) {
 			pr_warn("mapping->a_ops:%ps with invalid mapping->host inode address %px\n",
 				a_ops, host);
@@ -146,7 +146,7 @@ void __dump_page(struct page *page, const char *reason)
 		}
 
 		dentry_ptr = container_of(dentry_first, struct dentry, d_u.d_alias);
-		if (probe_kernel_read_strict(&dentry, dentry_ptr,
+		if (probe_kernel_read(&dentry, dentry_ptr,
 							sizeof(struct dentry))) {
 			pr_warn("mapping->aops:%ps with invalid mapping->host->i_dentry.first %px\n",
 				a_ops, dentry_ptr);
diff --git a/mm/maccess.c b/mm/maccess.c
index 85d124628b0e..253b786b2bf1 100644
--- a/mm/maccess.c
+++ b/mm/maccess.c
@@ -6,36 +6,13 @@
 #include <linux/mm.h>
 #include <linux/uaccess.h>
 
-static long __probe_kernel_read(void *dst, const void *src, size_t size,
-		bool strict);
-
-bool __weak probe_kernel_read_allowed(const void *unsafe_src, size_t size,
-		bool strict)
+bool __weak probe_kernel_read_allowed(const void *unsafe_src, size_t size)
 {
 	return true;
 }
 
 /**
- * probe_kernel_read(): safely attempt to read from any location
- * @dst: pointer to the buffer that shall take the data
- * @src: address to read from
- * @size: size of the data chunk
- *
- * Same as probe_kernel_read_strict() except that for architectures with
- * not fully separated user and kernel address spaces this function also works
- * for user address tanges.
- *
- * DO NOT USE THIS FUNCTION - it is broken on architectures with entirely
- * separate kernel and user address spaces, and also a bad idea otherwise.
- */
-long probe_kernel_read(void *dst, const void *src, size_t size)
-{
-	return __probe_kernel_read(dst, src, size, false);
-}
-EXPORT_SYMBOL_GPL(probe_kernel_read);
-
-/**
- * probe_kernel_read_strict(): safely attempt to read from kernel-space
+ * probe_kernel_read(): safely attempt to read from kernel-space
  * @dst: pointer to the buffer that shall take the data
  * @src: address to read from
  * @size: size of the data chunk
@@ -48,18 +25,12 @@ EXPORT_SYMBOL_GPL(probe_kernel_read);
  * probe_kernel_read() suitable for use within regions where the caller
  * already holds mmap_lock, or other locks which nest inside mmap_lock.
  */
-long probe_kernel_read_strict(void *dst, const void *src, size_t size)
-{
-	return __probe_kernel_read(dst, src, size, true);
-}
-
-static long __probe_kernel_read(void *dst, const void *src, size_t size,
-		bool strict)
+long probe_kernel_read(void *dst, const void *src, size_t size)
 {
 	long ret;
 	mm_segment_t old_fs = get_fs();
 
-	if (!probe_kernel_read_allowed(src, size, strict))
+	if (!probe_kernel_read_allowed(src, size))
 		return -EFAULT;
 
 	set_fs(KERNEL_DS);
@@ -73,6 +44,7 @@ static long __probe_kernel_read(void *dst, const void *src, size_t size,
 		return -EFAULT;
 	return 0;
 }
+EXPORT_SYMBOL_GPL(probe_kernel_read);
 
 /**
  * probe_user_read(): safely attempt to read from a user-space location
@@ -181,7 +153,7 @@ long strncpy_from_kernel_nofault(char *dst, const void *unsafe_addr, long count)
 
 	if (unlikely(count <= 0))
 		return 0;
-	if (!probe_kernel_read_allowed(unsafe_addr, count, true))
+	if (!probe_kernel_read_allowed(unsafe_addr, count))
 		return -EFAULT;
 
 	set_fs(KERNEL_DS);