[PATCH 9/9] fortify: Improve buffer overflow reporting

From: Kees Cook
Date: Wed Apr 05 2023 - 20:02:54 EST

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From: Kees Cook <kees@xxxxxxxxxxx>

Improve the reporting of buffer overflows under CONFIG_FORTIFY_SOURCE to
help accelerate debugging efforts. The calculations are all just sitting
in registers anyway, so pass them along to the function to be reported.

For example, before:

detected buffer overflow in memcpy

and after:

memcpy: detected buffer overflow: 4096 byte read from buffer of size 1

Signed-off-by: Kees Cook <kees@xxxxxxxxxxx>
---
include/linux/fortify-string.h | 60 +++++++++++++++++++---------------
lib/fortify_kunit.c | 4 +--
lib/string_helpers.c | 9 ++---
3 files changed, 40 insertions(+), 33 deletions(-)

diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
index 2bbee7b28e71..d37f4597cf68 100644
--- a/include/linux/fortify-string.h
+++ b/include/linux/fortify-string.h
@@ -14,8 +14,8 @@
#ifdef FORTIFY_KUNIT_OVERRIDE
# define fortify_panic kunit_fortify_panic
#else
-# define fortify_panic(func, write, retfail) \
- __fortify_panic(fortify_reason(func, write))
+# define fortify_panic(func, write, avail, size, retfail) \
+ __fortify_panic(fortify_reason(func, write), avail, size)
#endif

#define FORTIFY_READ 0
@@ -39,8 +39,8 @@
#define FORTIFY_FUNC_kmemdup 15
#define FORTIFY_FUNC_strcpy 16

-void __fortify_report(u8 reason);
-void __fortify_panic(u8 reason) __cold __noreturn;
+void __fortify_report(const u8 reason, const size_t avail, const size_t size);
+void __fortify_panic(const u8 reason, const size_t avail, const size_t size) __cold __noreturn;
void __read_overflow(void) __compiletime_error("detected read beyond size of object (1st parameter)");
void __read_overflow2(void) __compiletime_error("detected read beyond size of object (2nd parameter)");
void __read_overflow2_field(size_t avail, size_t wanted) __compiletime_warning("detected read beyond size of field (2nd parameter); maybe use struct_group()?");
@@ -178,7 +178,7 @@ char *strncpy(char * const POS p, const char *q, __kernel_size_t size)
if (__compiletime_lessthan(p_size, size))
__write_overflow();
if (p_size < size)
- fortify_panic(FORTIFY_FUNC_strncpy, FORTIFY_WRITE, p);
+ fortify_panic(FORTIFY_FUNC_strncpy, FORTIFY_WRITE, p_size, size, p);
return __underlying_strncpy(p, q, size);
}

@@ -209,7 +209,7 @@ __FORTIFY_INLINE __kernel_size_t strnlen(const char * const POS p, __kernel_size
/* Do not check characters beyond the end of p. */
ret = __real_strnlen(p, maxlen < p_size ? maxlen : p_size);
if (p_size <= ret && maxlen != ret)
- fortify_panic(FORTIFY_FUNC_strnlen, FORTIFY_READ, ret);
+ fortify_panic(FORTIFY_FUNC_strnlen, FORTIFY_READ, p_size, ret + 1, ret);
return ret;
}

@@ -245,7 +245,7 @@ __kernel_size_t __fortify_strlen(const char * const POS p)
return __underlying_strlen(p);
ret = strnlen(p, p_size);
if (p_size <= ret)
- fortify_panic(FORTIFY_FUNC_strlen, FORTIFY_READ, ret);
+ fortify_panic(FORTIFY_FUNC_strlen, FORTIFY_READ, p_size, ret + 1, ret);
return ret;
}

@@ -286,8 +286,8 @@ __FORTIFY_INLINE size_t strlcpy(char * const POS p, const char * const POS q, si
__write_overflow();
}
if (size) {
- if (len >= p_size)
- fortify_panic(FORTIFY_FUNC_strlcpy, FORTIFY_WRITE, q_len);
+ if (p_size <= len)
+ fortify_panic(FORTIFY_FUNC_strlcpy, FORTIFY_WRITE, p_size, len + 1, q_len);
__underlying_memcpy(p, q, len);
p[len] = '\0';
}
@@ -364,8 +364,8 @@ __FORTIFY_INLINE ssize_t strscpy(char * const POS p, const char * const POS q, s
* Generate a runtime write overflow error if len is greater than
* p_size.
*/
- if (len > p_size)
- fortify_panic(FORTIFY_FUNC_strscpy, FORTIFY_WRITE, -E2BIG);
+ if (p_size < len)
+ fortify_panic(FORTIFY_FUNC_strscpy, FORTIFY_WRITE, p_size, len, -E2BIG);

/*
* We can now safely call vanilla strscpy because we are protected from:
@@ -423,7 +423,7 @@ size_t strlcat(char * const POS p, const char * const POS q, size_t avail)

/* Give up if string is already overflowed. */
if (p_size <= p_len)
- fortify_panic(FORTIFY_FUNC_strlcat, FORTIFY_READ, wanted);
+ fortify_panic(FORTIFY_FUNC_strlcat, FORTIFY_READ, p_size, p_len + 1, wanted);

if (actual >= avail) {
copy_len = avail - p_len - 1;
@@ -432,7 +432,7 @@ size_t strlcat(char * const POS p, const char * const POS q, size_t avail)

/* Give up if copy will overflow. */
if (p_size <= actual)
- fortify_panic(FORTIFY_FUNC_strlcat, FORTIFY_WRITE, wanted);
+ fortify_panic(FORTIFY_FUNC_strlcat, FORTIFY_WRITE, p_size, actual + 1, wanted);
__underlying_memcpy(p + p_len, q, copy_len);
p[actual] = '\0';

@@ -459,9 +459,11 @@ __FORTIFY_INLINE __diagnose_as(__builtin_strcat, 1, 2)
char *strcat(char * const POS p, const char *q)
{
size_t p_size = __member_size(p);
+ size_t wanted;

- if (strlcat(p, q, p_size) >= p_size)
- fortify_panic(FORTIFY_FUNC_strcat, FORTIFY_WRITE, p);
+ wanted = strlcat(p, q, p_size);
+ if (p_size <= wanted)
+ fortify_panic(FORTIFY_FUNC_strcat, FORTIFY_WRITE, p_size, wanted + 1, p);
return p;
}

@@ -491,13 +493,15 @@ char *strncat(char * const POS p, const char * const POS q, __kernel_size_t coun
size_t p_len, copy_len;
size_t p_size = __member_size(p);
size_t q_size = __member_size(q);
+ size_t total;

if (p_size == SIZE_MAX && q_size == SIZE_MAX)
return __underlying_strncat(p, q, count);
p_len = strlen(p);
copy_len = strnlen(q, count);
- if (p_size < p_len + copy_len + 1)
- fortify_panic(FORTIFY_FUNC_strncat, FORTIFY_WRITE, p);
+ total = p_len + copy_len + 1;
+ if (p_size < total)
+ fortify_panic(FORTIFY_FUNC_strncat, FORTIFY_WRITE, p_size, total, p);
__underlying_memcpy(p + p_len, q, copy_len);
p[p_len + copy_len] = '\0';
return p;
@@ -538,7 +542,7 @@ __FORTIFY_INLINE bool fortify_memset_chk(__kernel_size_t size,
* lengths are unknown.)
*/
if (p_size != SIZE_MAX && p_size < size)
- fortify_panic(FORTIFY_FUNC_memset, FORTIFY_WRITE, true);
+ fortify_panic(FORTIFY_FUNC_memset, FORTIFY_WRITE, p_size, size, true);
return false;
}

@@ -638,9 +642,9 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size,
* lengths are unknown.)
*/
if (p_size != SIZE_MAX && p_size < size)
- fortify_panic(func, FORTIFY_WRITE, true);
+ fortify_panic(func, FORTIFY_WRITE, p_size, size, true);
else if (q_size != SIZE_MAX && q_size < size)
- fortify_panic(func, FORTIFY_READ, true);
+ fortify_panic(func, FORTIFY_READ, p_size, size, true);

/*
* Warn when writing beyond destination field size.
@@ -740,7 +744,7 @@ __FORTIFY_INLINE void *memscan(void * const POS0 p, int c, __kernel_size_t size)
if (__compiletime_lessthan(p_size, size))
__read_overflow();
if (p_size < size)
- fortify_panic(FORTIFY_FUNC_memscan, FORTIFY_READ, NULL);
+ fortify_panic(FORTIFY_FUNC_memscan, FORTIFY_READ, p_size, size, NULL);
return __real_memscan(p, c, size);
}

@@ -756,8 +760,10 @@ int memcmp(const void * const POS0 p, const void * const POS0 q, __kernel_size_t
if (__compiletime_lessthan(q_size, size))
__read_overflow2();
}
- if (p_size < size || q_size < size)
- fortify_panic(FORTIFY_FUNC_memcmp, FORTIFY_READ, INT_MIN);
+ if (p_size < size)
+ fortify_panic(FORTIFY_FUNC_memcmp, FORTIFY_READ, p_size, size, INT_MIN);
+ else if (q_size < size)
+ fortify_panic(FORTIFY_FUNC_memcmp, FORTIFY_READ, q_size, size, INT_MIN);
return __underlying_memcmp(p, q, size);
}

@@ -769,7 +775,7 @@ void *memchr(const void * const POS0 p, int c, __kernel_size_t size)
if (__compiletime_lessthan(p_size, size))
__read_overflow();
if (p_size < size)
- fortify_panic(FORTIFY_FUNC_memchr, FORTIFY_READ, NULL);
+ fortify_panic(FORTIFY_FUNC_memchr, FORTIFY_READ, p_size, size, NULL);
return __underlying_memchr(p, c, size);
}

@@ -781,7 +787,7 @@ __FORTIFY_INLINE void *memchr_inv(const void * const POS0 p, int c, size_t size)
if (__compiletime_lessthan(p_size, size))
__read_overflow();
if (p_size < size)
- fortify_panic(FORTIFY_FUNC_memchr_inv, FORTIFY_READ, NULL);
+ fortify_panic(FORTIFY_FUNC_memchr_inv, FORTIFY_READ, p_size, size, NULL);
return __real_memchr_inv(p, c, size);
}

@@ -794,7 +800,7 @@ __FORTIFY_INLINE void *kmemdup(const void * const POS0 p, size_t size, gfp_t gfp
if (__compiletime_lessthan(p_size, size))
__read_overflow();
if (p_size < size)
- fortify_panic(FORTIFY_FUNC_kmemdup, FORTIFY_READ, NULL);
+ fortify_panic(FORTIFY_FUNC_kmemdup, FORTIFY_READ, p_size, size, NULL);
return __real_kmemdup(p, size, gfp);
}

@@ -831,7 +837,7 @@ char *strcpy(char * const POS p, const char * const POS q)
__write_overflow();
/* Run-time check for dynamic size overflow. */
if (p_size < size)
- fortify_panic(FORTIFY_FUNC_strcpy, FORTIFY_WRITE, p);
+ fortify_panic(FORTIFY_FUNC_strcpy, FORTIFY_WRITE, p_size, size, p);
__underlying_memcpy(p, q, size);
return p;
}
diff --git a/lib/fortify_kunit.c b/lib/fortify_kunit.c
index b7c884037629..b022797c9fe6 100644
--- a/lib/fortify_kunit.c
+++ b/lib/fortify_kunit.c
@@ -18,9 +18,9 @@
/* Call kunit_fortify_panic() instead of fortify_panic() */
#define FORTIFY_KUNIT_OVERRIDE
void fortify_add_kunit_error(int write);
-#define kunit_fortify_panic(func, write, retfail) \
+#define kunit_fortify_panic(func, write, avail, size, retfail) \
do { \
- __fortify_report(fortify_reason(func, write)); \
+ __fortify_report(fortify_reason(func, write), avail, size); \
fortify_add_kunit_error(write); \
return (retfail); \
} while (0)
diff --git a/lib/string_helpers.c b/lib/string_helpers.c
index 5bb65f623e40..cc15a25556fb 100644
--- a/lib/string_helpers.c
+++ b/lib/string_helpers.c
@@ -1023,7 +1023,7 @@ EXPORT_SYMBOL(__read_overflow2_field);
void __write_overflow_field(size_t avail, size_t wanted) { }
EXPORT_SYMBOL(__write_overflow_field);

-void __fortify_report(u8 reason)
+void __fortify_report(const u8 reason, const size_t avail, const size_t size)
{
const char *name;
const bool write = !!(reason & 0x1);
@@ -1083,13 +1083,14 @@ void __fortify_report(u8 reason)
default:
name = "unknown";
}
- WARN(1, "%s: detected buffer %s overflow\n", name, write ? "write" : "read");
+ WARN(1, "%s: detected buffer overflow: %zu byte %s buffer of size %zu\n",
+ name, size, write ? "write to" : "read from", avail);
}
EXPORT_SYMBOL(__fortify_report);

-void __fortify_panic(const u8 reason)
+void __fortify_panic(const u8 reason, const size_t avail, const size_t size)
{
- __fortify_report(reason);
+ __fortify_report(reason, avail, size);
BUG();
}
EXPORT_SYMBOL(__fortify_panic);
--
2.34.1

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Next in thread: Kees Cook: "[PATCH 8/9] fortify: Add KUnit tests for runtime overflows"
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