Re: [PATCH] exofs: Avoid VLA in structures

From: Nick Desaulniers
Date: Thu Mar 29 2018 - 17:32:34 EST

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On Tue, Mar 27, 2018 at 1:39 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:

> On the quest to remove all VLAs from the kernel[1] this adjusts several
> cases where allocation is made after an array of structures that points
> back into the allocation. The allocations are changed to perform explicit
> calculations instead of using a Variable Length Array in a structure.
> Additionally, this lets Clang compile this code now, since Clang does not
> support VLAIS[2].

> [1] https://lkml.org/lkml/2018/3/7/621
> [2] https://lkml.org/lkml/2013/9/23/500

> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
> I not sure the best way to test this. Kconfig implies I need special
hardware?
> ---
> fs/exofs/ore.c | 84
+++++++++++++++++++++++++++++++----------------------
> fs/exofs/ore_raid.c | 73 ++++++++++++++++++++++++++++++++++------------
> fs/exofs/super.c | 23 +++++++--------
> 3 files changed, 114 insertions(+), 66 deletions(-)

> diff --git a/fs/exofs/ore.c b/fs/exofs/ore.c
> index 3c6a9c156b7a..cfa862ea19d2 100644
> --- a/fs/exofs/ore.c
> +++ b/fs/exofs/ore.c
> @@ -146,68 +146,82 @@ int _ore_get_io_state(struct ore_layout *layout,
> struct ore_io_state **pios)
> {
> struct ore_io_state *ios;
> - struct page **pages;
> - struct osd_sg_entry *sgilist;
> + size_t size_ios, size_extra, size_total;
> + void *ios_extra;
> +
> + /*
> + * The desired layout looks like this, with the extra_allocation
> + * items pointed at from fields within ios or per_dev:
> +
> struct __alloc_all_io_state {
> struct ore_io_state ios;
> struct ore_per_dev_state per_dev[numdevs];
> union {
> struct osd_sg_entry sglist[sgs_per_dev * numdevs];
> struct page *pages[num_par_pages];
> - };
> - } *_aios;
> -
> - if (likely(sizeof(*_aios) <= PAGE_SIZE)) {
> - _aios = kzalloc(sizeof(*_aios), GFP_KERNEL);
> - if (unlikely(!_aios)) {
> - ORE_DBGMSG("Failed kzalloc bytes=%zd\n",
> - sizeof(*_aios));
> + } extra_allocation;
> + } whole_allocation;
> +
> + */
> +
> + /* This should never happen, so abort early if it ever does. */
> + if (sgs_per_dev && num_par_pages) {
> + ORE_DBGMSG("Tried to use both pages and sglist\n");
> + *pios = NULL;
> + return -EINVAL;
> + }
> +
> + if (numdevs > (INT_MAX - sizeof(*ios)) /
> + sizeof(struct ore_per_dev_state))
> + return -ENOMEM;
> + size_ios = sizeof(*ios) + sizeof(struct ore_per_dev_state) *
numdevs;

Can all these invariant checks that return -ENOMEM be grouped together, as
it seems we have ...

> +
> + if (sgs_per_dev * numdevs > INT_MAX / sizeof(struct osd_sg_entry))
> + return -ENOMEM;
> + if (num_par_pages > INT_MAX / sizeof(struct page *))
> + return -ENOMEM;

...a whole bunch of single conditions with the same body, can be combined
with one:

if (a)
return d;
if (b)
return d;
if (c)
return d;

->

if (a || b || c)
return d;

> + size_extra = max(sizeof(struct osd_sg_entry) * (sgs_per_dev *
numdevs),

Do you need parens around the sub-expression `(sgs_per_dev * numdevs)`?

> + sizeof(struct page *) * num_par_pages);
> +
> + size_total = size_ios + size_extra;
> +
> + if (likely(size_total <= PAGE_SIZE)) {
> + ios = kzalloc(size_total, GFP_KERNEL);
> + if (unlikely(!ios)) {
> + ORE_DBGMSG("Failed kzalloc bytes=%zd\n",
size_total);
> *pios = NULL;
> return -ENOMEM;
> }
> - pages = num_par_pages ? _aios->pages : NULL;
> - sgilist = sgs_per_dev ? _aios->sglist : NULL;
> - ios = &_aios->ios;
> + ios_extra = (char *)ios + size_ios;
> } else {
> - struct __alloc_small_io_state {
> - struct ore_io_state ios;
> - struct ore_per_dev_state per_dev[numdevs];
> - } *_aio_small;
> - union __extra_part {
> - struct osd_sg_entry sglist[sgs_per_dev * numdevs];
> - struct page *pages[num_par_pages];
> - } *extra_part;
> -
> - _aio_small = kzalloc(sizeof(*_aio_small), GFP_KERNEL);
> - if (unlikely(!_aio_small)) {
> + ios = kzalloc(size_ios, GFP_KERNEL);
> + if (unlikely(!ios)) {
> ORE_DBGMSG("Failed alloc first part bytes=%zd\n",
> - sizeof(*_aio_small));
> + size_ios);
> *pios = NULL;
> return -ENOMEM;
> }
> - extra_part = kzalloc(sizeof(*extra_part), GFP_KERNEL);
> - if (unlikely(!extra_part)) {
> + ios_extra = kzalloc(size_extra, GFP_KERNEL);
> + if (unlikely(!ios_extra)) {
> ORE_DBGMSG("Failed alloc second part bytes=%zd\n",
> - sizeof(*extra_part));
> - kfree(_aio_small);
> + size_extra);
> + kfree(ios);
> *pios = NULL;
> return -ENOMEM;
> }

> - pages = num_par_pages ? extra_part->pages : NULL;
> - sgilist = sgs_per_dev ? extra_part->sglist : NULL;
> /* In this case the per_dev[0].sgilist holds the pointer
to
> * be freed
> */
> - ios = &_aio_small->ios;
> ios->extra_part_alloc = true;
> }

> - if (pages) {
> - ios->parity_pages = pages;
> + if (num_par_pages) {
> + ios->parity_pages = ios_extra;
> ios->max_par_pages = num_par_pages;
> }
> - if (sgilist) {
> + if (sgs_per_dev) {
> + struct osd_sg_entry *sgilist = ios_extra;
> unsigned d;

> for (d = 0; d < numdevs; ++d) {
> diff --git a/fs/exofs/ore_raid.c b/fs/exofs/ore_raid.c
> index 27cbdb697649..659129d5e9f7 100644
> --- a/fs/exofs/ore_raid.c
> +++ b/fs/exofs/ore_raid.c
> @@ -71,6 +71,11 @@ static int _sp2d_alloc(unsigned pages_in_unit,
unsigned group_width,
> {
> struct __stripe_pages_2d *sp2d;
> unsigned data_devs = group_width - parity;
> +
> + /*
> + * Desired allocation layout is, though when larger than
PAGE_SIZE,
> + * each struct __alloc_1p_arrays is separately allocated:
> +
> struct _alloc_all_bytes {
> struct __alloc_stripe_pages_2d {
> struct __stripe_pages_2d sp2d;
> @@ -82,55 +87,85 @@ static int _sp2d_alloc(unsigned pages_in_unit,
unsigned group_width,
> char page_is_read[data_devs];
> } __a1pa[pages_in_unit];
> } *_aab;
> +
> struct __alloc_1p_arrays *__a1pa;
> struct __alloc_1p_arrays *__a1pa_end;
> - const unsigned sizeof__a1pa = sizeof(_aab->__a1pa[0]);
> +
> + */
> +
> + char *__a1pa;
> + char *__a1pa_end;
> +

Just my notes, since this block could use another eye for review:

> + const size_t sizeof_stripe_pages_2d =
> + sizeof(struct __stripe_pages_2d) +
> + sizeof(struct __1_page_stripe) * pages_in_unit;

Ok, so this replaces sizeof(_aab->__asp2d).

> + const size_t sizeof__a1pa =
> + ALIGN(sizeof(struct page *) * (2 * group_width) +
data_devs,
> + sizeof(void *));

And this replaces sizeof(_aab->__a1pa[0]);

> + const size_t sizeof__a1pa_arrays = sizeof__a1pa * pages_in_unit;

This replaces sizeof(_aab->__a1pa[pages_in_unit]);

> + const size_t alloc_total = sizeof_stripe_pages_2d +
> + sizeof__a1pa_arrays;

Replaces sizeof(*_aab);

This is the trickiest part of this patch IMO, I think it needs careful
review, but looks correct to me.

> +
> unsigned num_a1pa, alloc_size, i;

> /* FIXME: check these numbers in ore_verify_layout */
> - BUG_ON(sizeof(_aab->__asp2d) > PAGE_SIZE);
> + BUG_ON(sizeof_stripe_pages_2d > PAGE_SIZE);
> BUG_ON(sizeof__a1pa > PAGE_SIZE);

> - if (sizeof(*_aab) > PAGE_SIZE) {
> - num_a1pa = (PAGE_SIZE - sizeof(_aab->__asp2d)) /
sizeof__a1pa;
> - alloc_size = sizeof(_aab->__asp2d) + sizeof__a1pa *
num_a1pa;
> + /*
> + * If alloc_total would be larger than PAGE_SIZE, only allocate
> + * as many a1pa items as would fill the rest of the page, instead
> + * of the full pages_in_unit count.
> + */
> + if (alloc_total > PAGE_SIZE) {
> + num_a1pa = (PAGE_SIZE - sizeof_stripe_pages_2d) /
sizeof__a1pa;
> + alloc_size = sizeof_stripe_pages_2d + sizeof__a1pa *
num_a1pa;
> } else {
> num_a1pa = pages_in_unit;
> - alloc_size = sizeof(*_aab);
> + alloc_size = alloc_total;
> }

> - _aab = kzalloc(alloc_size, GFP_KERNEL);
> - if (unlikely(!_aab)) {
> + *psp2d = sp2d = kzalloc(alloc_size, GFP_KERNEL);
> + if (unlikely(!sp2d)) {
> ORE_DBGMSG("!! Failed to alloc sp2d size=%d\n",
alloc_size);
> return -ENOMEM;
> }
> + /* From here Just call _sp2d_free */

> - sp2d = &_aab->__asp2d.sp2d;
> - *psp2d = sp2d; /* From here Just call _sp2d_free */
> -
> - __a1pa = _aab->__a1pa;
> - __a1pa_end = __a1pa + num_a1pa;
> + /* Find start of a1pa area. */
> + __a1pa = (char *)sp2d + sizeof_stripe_pages_2d;
> + /* Find end of the _allocated_ a1pa area. */
> + __a1pa_end = __a1pa + alloc_size;

> + /* Allocate additionally needed a1pa items in PAGE_SIZE chunks. */
> for (i = 0; i < pages_in_unit; ++i) {
> if (unlikely(__a1pa >= __a1pa_end)) {
> num_a1pa = min_t(unsigned, PAGE_SIZE /
sizeof__a1pa,
> pages_in_unit -
i);
> + alloc_size = sizeof__a1pa * num_a1pa;

> - __a1pa = kcalloc(num_a1pa, sizeof__a1pa,
GFP_KERNEL);
> + __a1pa = kzalloc(alloc_size, GFP_KERNEL);
> if (unlikely(!__a1pa)) {
> ORE_DBGMSG("!! Failed to
_alloc_1p_arrays=%d\n",
> num_a1pa);
> return -ENOMEM;
> }
> - __a1pa_end = __a1pa + num_a1pa;
> + __a1pa_end = __a1pa + alloc_size;
> /* First *pages is marked for kfree of the buffer
*/
> sp2d->_1p_stripes[i].alloc = true;
> }

> - sp2d->_1p_stripes[i].pages = __a1pa->pages;
> - sp2d->_1p_stripes[i].scribble = __a1pa->scribble ;
> - sp2d->_1p_stripes[i].page_is_read = __a1pa->page_is_read;
> - ++__a1pa;
> + /*
> + * Attach all _lp_stripes pointers to the allocation for
> + * it which was either part of the original PAGE_SIZE
> + * allocation or the subsequent allocation in this loop.
> + */
> + sp2d->_1p_stripes[i].pages = (void *)__a1pa;
> + sp2d->_1p_stripes[i].scribble =
> + sp2d->_1p_stripes[i].pages + group_width;
> + sp2d->_1p_stripes[i].page_is_read =
> + (char *)(sp2d->_1p_stripes[i].scribble +
group_width);

Can you DRY up `sp2d->_1p_stripes[i]`? ex.

struct _1p_stripes* stripe;

for (i = 0; i < pages_in_unit; ...
...
stripe = &sp2d->_1p_stripes[i];
stripe->pages = (void*)__a1pa;
stripe->scribble = stripe->pages + group_width;
stripe->page_is_read = (char*)stripe->scribble + group_width;

> + __a1pa += sizeof__a1pa;
> }

> sp2d->parity = parity;
> diff --git a/fs/exofs/super.c b/fs/exofs/super.c
> index 179cd5c2f52a..f3c29e9326f1 100644
> --- a/fs/exofs/super.c
> +++ b/fs/exofs/super.c
> @@ -549,27 +549,26 @@ static int exofs_devs_2_odi(struct
exofs_dt_device_info *dt_dev,
> static int __alloc_dev_table(struct exofs_sb_info *sbi, unsigned numdevs,
> struct exofs_dev **peds)
> {
> - struct __alloc_ore_devs_and_exofs_devs {
> - /* Twice bigger table: See exofs_init_comps() and comment
at
> - * exofs_read_lookup_dev_table()
> - */
> - struct ore_dev *oreds[numdevs * 2 - 1];
> - struct exofs_dev eds[numdevs];
> - } *aoded;
> + /* Twice bigger table: See exofs_init_comps() and comment at
> + * exofs_read_lookup_dev_table()
> + */
> + size_t numores = numdevs * 2 - 1;

const size_t

> struct exofs_dev *eds;
> unsigned i;

> - aoded = kzalloc(sizeof(*aoded), GFP_KERNEL);
> - if (unlikely(!aoded)) {
> + sbi->oc.ods = kzalloc(numores * sizeof(struct ore_dev *) +
> + numdevs * sizeof(struct exofs_dev),
GFP_KERNEL);
> + if (unlikely(!sbi->oc.ods)) {
> EXOFS_ERR("ERROR: failed allocating Device array[%d]\n",
> numdevs);
> return -ENOMEM;
> }

> - sbi->oc.ods = aoded->oreds;
> - *peds = eds = aoded->eds;
> + /* Start of allocated struct exofs_dev entries */
> + *peds = eds = (void *)sbi->oc.ods[numores];
> + /* Initialize pointers into struct exofs_dev */
> for (i = 0; i < numdevs; ++i)
> - aoded->oreds[i] = &eds[i].ored;
> + sbi->oc.ods[i] = &eds[i].ored;
> return 0;
> }

> --
> 2.7.4

> --
> Kees Cook
> Pixel Security

The sizeof calculations replacing the VLAIS I pointed out are pretty tricky
to follow, but I feel pretty confident about this patch. With the changes
I recommend, feel free to add my Reviewed-by tag. It would be good to get
additional review and testing from the maintainer.

Thanks for taking the time to tackle this.

--
Thanks,
~Nick Desaulniers

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