Re: [PATCH] x86/uv/time: Replace one-element array and save heap space

Gustavo A. R. Silva
2020-05-21T23:19:28Z

[+CC John Stultz <john.stultz@linaro.org> and +Kees' Reviewed-by tag]

Reviewed-by: Kees Cook <keescook@chromium.org>

On Mon, May 18, 2020 at 02:01:14PM -0500, Gustavo A. R. Silva wrote:
> The current codebase makes use of one-element arrays in the following
> form:
>
> struct something {
> int length;
> u8 data[1];
> };
>
> struct something *instance;
>
> instance = kmalloc(sizeof(*instance) + size, GFP_KERNEL);
> instance->length = size;
> memcpy(instance->data, source, size);
>
> but the preferred mechanism to declare variable-length types such as
> these ones is a flexible array member[1][2], introduced in C99:
>
> struct foo {
> int stuff;
> struct boo array[];
> };
>
> By making use of the mechanism above, we will get a compiler warning
> in case the flexible array does not occur last in the structure, which
> will help us prevent some kind of undefined behavior bugs from being
> inadvertently introduced[3] to the codebase from now on. So, replace
> the one-element array with a flexible-array member.
>
> Also, make use of the new struct_size() helper to properly calculate the
> total size needed to allocate dynamic memory for struct uv_rtc_timer_head.
> Notice that, due to the use of a one-element array, space for an extra
> struct cpu:
>
> struct {
> int lcpu; /* systemwide logical cpu number */
> u64 expires; /* next timer expiration for this cpu */
> } cpu[1]
>
> was being allocated at the moment of applying the sizeof operator to
> struct uv_rtc_timer_head in the call to kmalloc_node() at line 159:
>
> 159 head = kmalloc_node(sizeof(struct uv_rtc_timer_head) +
> 160 (uv_blade_nr_possible_cpus(bid) *
> 161 2 * sizeof(u64)),
> 162 GFP_KERNEL, nid);
>
> but that extra cpu[] was never actually being accessed due to the
> following piece of code at line 168:
>
> 168 head->ncpus = uv_blade_nr_possible_cpus(bid);
>
> and the piece of code at line 187:
>
> 187 for (c = 0; c < head->ncpus; c++) {
> 188 u64 exp = head->cpu[c].expires;
> 189 if (exp < lowest) {
> 190 bcpu = c;
> 191 lowest = exp;
> 192 }
> 193 }
>
> so heap space was being wasted.
>
> Another thing important to notice is that through the use of the
> struct_size() helper, code at line 161:
>
> 161 2 * sizeof(u64)),
>
> is changed to now be the actual size of struct cpu; see
> sizeof(*(p)->member) at include/linux/overflow.h:314:
>
> 314 #define struct_size(p, member, n) \
> 315 __ab_c_size(n, \
> 316 sizeof(*(p)->member) + __must_be_array((p)->member),\
> 317 sizeof(*(p)))
>
> As a side note, the original developer could have implemented code at line
> 161: 2 * sizeof(64) as follows:
>
> sizeof(*head->cpu)
>
> This issue has been out there since 2009.
>
> This issue was found with the help of Coccinelle and fixed _manually_.
>
> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> [2] https://github.com/KSPP/linux/issues/21
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
>
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> ---
> arch/x86/platform/uv/uv_time.c | 7 +++----
> 1 file changed, 3 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/platform/uv/uv_time.c b/arch/x86/platform/uv/uv_time.c
> index 7af31b245636..993a8ae6fdfb 100644
> --- a/arch/x86/platform/uv/uv_time.c
> +++ b/arch/x86/platform/uv/uv_time.c
> @@ -52,7 +52,7 @@ struct uv_rtc_timer_head {
> struct {
> int lcpu; /* systemwide logical cpu number */
> u64 expires; /* next timer expiration for this cpu */
> - } cpu[1];
> + } cpu[];
> };
>
> /*
> @@ -156,9 +156,8 @@ static __init int uv_rtc_allocate_timers(void)
> struct uv_rtc_timer_head *head = blade_info[bid];
>
> if (!head) {
> - head = kmalloc_node(sizeof(struct uv_rtc_timer_head) +
> - (uv_blade_nr_possible_cpus(bid) *
> - 2 * sizeof(u64)),
> + head = kmalloc_node(struct_size(head, cpu,
> + uv_blade_nr_possible_cpus(bid)),
> GFP_KERNEL, nid);
> if (!head) {
> uv_rtc_deallocate_timers();
> --
> 2.26.2
>