Re: [PATCH v3] perf/ring_buffer: Prefer struct_size over open coded arithmetic

[Kees Cook]

On Sat, May 11, 2024 at 03:24:37PM +0200, Erick Archer wrote:
> This is an effort to get rid of all multiplications from allocation
> functions in order to prevent integer overflows [1][2].
> 
> As the "rb" variable is a pointer to "struct perf_buffer" and this
> structure ends in a flexible array:
> 
> struct perf_buffer {
> 	[...]
> 	void	*data_pages[];
> };
> 
> the preferred way in the kernel is to use the struct_size() helper to
> do the arithmetic instead of the calculation "size + count * size" in
> the kzalloc_node() functions.
> 
> In the "rb_alloc" function defined in the else branch of the macro
> 
>  #ifndef CONFIG_PERF_USE_VMALLOC
> 
> the count in the struct_size helper is the literal "1" since only one
> pointer to void is allocated. Also, remove the "size" variable as it
> is no longer needed.
> 
> At the same time, prepare for the coming implementation by GCC and Clang
> of the __counted_by attribute. Flexible array members annotated with
> __counted_by can have their accesses bounds-checked at run-time via
> CONFIG_UBSAN_BOUNDS (for array indexing) and CONFIG_FORTIFY_SOURCE (for
> strcpy/memcpy-family functions). In this case, it is important to note
> that the logic needs a little refactoring to ensure that the "nr_pages"
> member is initialized before the first access to the flex array.
> 
> In one case, it is only necessary to move the "nr_pages" assignment
> before the array-writing loop while in the other case the access to the
> flex array needs to be moved inside the if branch and after the
> "nr_pages" assignment.
> 
> This way, the code is more safer.
> 
> This code was detected with the help of Coccinelle, and audited and
> modified manually.
> 
> Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments [1]
> Link: https://github.com/KSPP/linux/issues/160 [2]
> Signed-off-by: Erick Archer <erick.archer@xxxxxxxxxxx>

Thanks!

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-- 
Kees Cook