[PATCH mm] maccess: fix strncpy_from_user_nofault empty string handling

[Mykyta Yatsenko <mykyta.yatsenko5@gmail.com>]

From: Mykyta Yatsenko <yatsenko@meta.com>

strncpy_from_user_nofault should return the length of the copied string
including the trailing NUL, but if the argument unsafe_addr points to
an empty string ({'\0'}), the return value is 0.

This happens as strncpy_from_user copies terminal symbol into dst
and returns 0 (as expected), but strncpy_from_user_nofault does not
modify ret as it is not equal to count and not greater than 0, so 0 is
returned, which contradicts the contract.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
---
 kernel/trace/trace_events_filter.c | 10 ++++++++--
 mm/maccess.c                       |  2 +-
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c
index 0993dfc1c5c1..86b7e5a4e235 100644
--- a/kernel/trace/trace_events_filter.c
+++ b/kernel/trace/trace_events_filter.c
@@ -800,6 +800,7 @@ static __always_inline char *test_string(char *str)
 {
 	struct ustring_buffer *ubuf;
 	char *kstr;
+	int cnt;
 
 	if (!ustring_per_cpu)
 		return NULL;
@@ -808,7 +809,9 @@ static __always_inline char *test_string(char *str)
 	kstr = ubuf->buffer;
 
 	/* For safety, do not trust the string pointer */
-	if (!strncpy_from_kernel_nofault(kstr, str, USTRING_BUF_SIZE))
+	cnt = strncpy_from_kernel_nofault(kstr, str, USTRING_BUF_SIZE);
+	/* Return null if empty string or error */
+	if (cnt <= 1)
 		return NULL;
 	return kstr;
 }
@@ -818,6 +821,7 @@ static __always_inline char *test_ustring(char *str)
 	struct ustring_buffer *ubuf;
 	char __user *ustr;
 	char *kstr;
+	int cnt;
 
 	if (!ustring_per_cpu)
 		return NULL;
@@ -827,7 +831,9 @@ static __always_inline char *test_ustring(char *str)
 
 	/* user space address? */
 	ustr = (char __user *)str;
-	if (!strncpy_from_user_nofault(kstr, ustr, USTRING_BUF_SIZE))
+	cnt = strncpy_from_user_nofault(kstr, ustr, USTRING_BUF_SIZE);
+	/* Return null if empty string or error */
+	if (cnt <= 1)
 		return NULL;
 
 	return kstr;
diff --git a/mm/maccess.c b/mm/maccess.c
index 8f0906180a94..831b4dd7296c 100644
--- a/mm/maccess.c
+++ b/mm/maccess.c
@@ -196,7 +196,7 @@ long strncpy_from_user_nofault(char *dst, const void __user *unsafe_addr,
 	if (ret >= count) {
 		ret = count;
 		dst[ret - 1] = '\0';
-	} else if (ret > 0) {
+	} else if (ret >= 0) {
 		ret++;
 	}
 
-- 
2.49.0