RE: [PATCH 5/5] crypto: stm32/crc: protect from concurrent accesses

[Nicolas TOROMANOFF]

> -----Original Message-----
> From: Ard Biesheuvel <ardb@xxxxxxxxxx>
> Sent: Monday, May 25, 2020 11:07 AM
> To: Nicolas TOROMANOFF <nicolas.toromanoff@xxxxxx>; Eric Biggers
> <ebiggers@xxxxxxxxxx>
> On Mon, 25 May 2020 at 11:01, Nicolas TOROMANOFF
> <nicolas.toromanoff@xxxxxx> wrote:
> >
> > > -----Original Message-----
> > > From: Ard Biesheuvel <ardb@xxxxxxxxxx>
> > > Sent: Monday, May 25, 2020 9:46 AM
> > > To: Nicolas TOROMANOFF <nicolas.toromanoff@xxxxxx>
> > > Subject: Re: [PATCH 5/5] crypto: stm32/crc: protect from concurrent
> > > accesses
> > >
> > > On Mon, 25 May 2020 at 09:24, Nicolas TOROMANOFF
> > > <nicolas.toromanoff@xxxxxx> wrote:
> > > >
> > > > Hello,
> > > >
> > > > > -----Original Message-----
> > > > > From: Ard Biesheuvel <ardb@xxxxxxxxxx>
> > > > > Sent: Friday, May 22, 2020 6:12 PM> On Tue, 12 May 2020 at
> > > > > 16:13, Nicolas Toromanoff <nicolas.toromanoff@xxxxxx> wrote:
> > > > > >
> > > > > > Protect STM32 CRC device from concurrent accesses.
> > > > > >
> > > > > > As we create a spinlocked section that increase with buffer
> > > > > > size, we provide a module parameter to release the pressure by
> > > > > > splitting critical section in chunks.
> > > > > >
> > > > > > Size of each chunk is defined in burst_size module parameter.
> > > > > > By default burst_size=0, i.e. don't split incoming buffer.
> > > > > >
> > > > > > Signed-off-by: Nicolas Toromanoff <nicolas.toromanoff@xxxxxx>
> > > > >
> > > > > Would you mind explaining the usage model here? It looks like
> > > > > you are sharing a CRC hardware accelerator with a synchronous
> > > > > interface between different users by using spinlocks? You are
> > > > > aware that this will tie up the waiting CPUs completely during
> > > > > this time, right? So it would be much better to use a mutex
> > > > > here. Or perhaps it would make more sense to fall back to a s/w
> > > > > based CRC routine if the h/w is tied up
> > > working for another task?
> > > >
> > > > I know mutex are more acceptable here, but shash _update() and
> > > > _init() may be call from any context, and so I cannot take a mutex.
> > > > And to protect my concurrent HW access I only though about spinlock.
> > > > Due to possible constraint on CPUs, I add a burst_size option to
> > > > force slitting long buffer into smaller one, and so decrease time we take
> the lock.
> > > > But I didn't though to fallback to software CRC.
> > > >
> > > > I'll do a patch on top.
> > > > In in the burst_update() function I'll use a
> > > > spin_trylock_irqsave() and use
> > > software CRC32 if HW is already in use.
> > > >
> > >
> > > Right. I didn't even notice that you were keeping interrupts
> > > disabled the whole time when using the h/w block. That means that
> > > any serious use of this h/w block will make IRQ latency go through the roof.
> > >
> > > I recommend that you go back to the drawing board on this driver,
> > > rather than papering over the issues with a spin_trylock(). Perhaps
> > > it would be better to model it as a ahash (even though the h/w block
> > > itself is synchronous) and use a kthread to feed in the data.
> >
> > I thought when I updated the driver to move to a ahash interface, but
> > the main usage of crc32 is the ext4 fs, that calls the shash API.
> > Commit 877b5691f27a ("crypto: shash - remove shash_desc::flags")
> > removed possibility to sleep in shash callback. (before this commit
> > and with MAY_SLEEP option set, using a mutex may have been fine).
> >
> 
> According to that commit's log, sleeping is never fine for shash(), since it uses
> kmap_atomic() when iterating over the scatterlist.

Today, we could avoid using kmap_atomic() in shash_ashash_*() APIs (the
ones that Walks through the scatterlist) by using the
crypto_ahash_walk_first() function to initialize the shash_ahash walker
(note that this function is never call in current kernel source [to remove ?]).
Then shash_ahash_*() functions will call ahash_*() function that use kmap()
if (walk->flags & CRYPTO_ALG_ASYNC) [flag set by crypto_ahash_walk_first()]
The last kmap_atomic() will be in the shash_ahash_digest() call in the
optimize branch (that should be replaced by the no atomic one)

I didn't investigate more this way, because I didn't check the drawback of
using kmap() instead of kmap_atomic(), I wanted to avoid modifying behavior
of other drivers and using a function never use elsewhere in kernel scarred
me ;-).
If these updates correct visible bugs in the ahash usage of the stm32-crc32
code [no more "sleep while atomic" traces even with mutex in tests], 
Documentation states that shash API could be called from any context,
I cannot add mutex in them.

> > By now the solution I see is to use the spin_trylock_irqsave(),
> > fallback to software crc *AND* capping burst_size to ensure the locked
> section stay reasonable.
> >
> > Does this seems acceptable ?
> >
> 
> If the reason for disabling interrupts is to avoid deadlocks, wouldn't the switch
> to trylock() with a software fallback allow us to keep interrupts enabled?

Right, with the trylock, I don't see why we may need to mask interrupts.