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Subject[PATCH v2 1/3] vsprintf: Remove accidental VLA usage
Date
In the quest to remove all stack VLAs from the kernel[1], this introduces
a new "simple max" macro, and changes the "sym" array size calculation to
use it. The value is actually a fixed size, but since the max() macro uses
some extensive tricks for safety, it ends up looking like a variable size
to the compiler.

[1] https://lkml.org/lkml/2018/3/7/621

Signed-off-by: Kees Cook <keescook@chromium.org>
---
include/linux/kernel.h | 11 +++++++++++
lib/vsprintf.c | 4 ++--
2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 3fd291503576..1da554e9997f 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -820,6 +820,17 @@ static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { }
x, y)

/**
+ * SIMPLE_MAX - return maximum of two values without any type checking
+ * @x: first value
+ * @y: second value
+ *
+ * This should only be used in stack array sizes, since the type-checking
+ * from max() confuses the compiler into thinking a VLA is being used.
+ */
+#define SIMPLE_MAX(x, y) ((size_t)(x) > (size_t)(y) ? (size_t)(x) \
+ : (size_t)(y))
+
+/**
* min3 - return minimum of three values
* @x: first value
* @y: second value
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index d7a708f82559..50cce36e1cdc 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -744,8 +744,8 @@ char *resource_string(char *buf, char *end, struct resource *res,
#define FLAG_BUF_SIZE (2 * sizeof(res->flags))
#define DECODED_BUF_SIZE sizeof("[mem - 64bit pref window disabled]")
#define RAW_BUF_SIZE sizeof("[mem - flags 0x]")
- char sym[max(2*RSRC_BUF_SIZE + DECODED_BUF_SIZE,
- 2*RSRC_BUF_SIZE + FLAG_BUF_SIZE + RAW_BUF_SIZE)];
+ char sym[SIMPLE_MAX(2*RSRC_BUF_SIZE + DECODED_BUF_SIZE,
+ 2*RSRC_BUF_SIZE + FLAG_BUF_SIZE + RAW_BUF_SIZE)];

char *p = sym, *pend = sym + sizeof(sym);
int decode = (fmt[0] == 'R') ? 1 : 0;
--
2.7.4
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