Re: [PATCH] crypto: rsa: add debug message if leading zero byte is missing

[Ignat Korchagin]

On Wed, Feb 18, 2026 at 9:36 AM Kepplinger-Novakovic Martin
<Martin.Kepplinger-Novakovic@xxxxxxxxxxxxx> wrote:
>
> Am Donnerstag, dem 12.02.2026 um 11:15 +0000 schrieb Ignat Korchagin:
> > Hi,
> >
> > On Thu, Feb 12, 2026 at 10:39 AM Martin Kepplinger-Novakovic
> > <martin.kepplinger-novakovic@xxxxxxxxxxxxx> wrote:
> > >
> > > When debugging RSA certificate validation it can be valuable to see
> > > why the RSA verify() callback returns -EINVAL.
> >
> > Not sure if this would be some kind of an information leak (depending
> > on a subsystem using RSA). Also what makes this case so special?
> > Should we then annotate every other validation check in the code?
> >
> > > Signed-off-by: Martin Kepplinger-Novakovic
> > > <martin.kepplinger-novakovic@xxxxxxxxxxxxx>
> > > ---
> > >
> > > hi,
> > >
> > > my real issue is: When using a certificate based on an RSA-key,
> > > I sometimes see signature-verify errors and (via dm-verity)
> > > rootfs signature-verify errors, all triggered by "no leading 0
> > > byte".
> > >
> > > key/cert generation:
> > > openssl req -x509 -newkey rsa:4096 -keyout ca_key.pem -out ca.pem -
> > > nodes -days 365 -set_serial 01 -subj /CN=ginzinger.com
> > > and simply used as trusted built-in key and rootfs hash sign
> > > appended
> > > to rootfs (squashfs).
> > >
> > > I'm on imx6ul. The thing is: Using the same certificate/key, works
> > > on
> > > old v5.4-based kernels, up to v6.6!
> > >
> > > Starting with commit 2f1f34c1bf7b309 ("crypto: ahash - optimize
> > > performance
> > > when wrapping shash") it starts to break. it is not a commit on
> > > it's own I
> > > can revert and move on.
> > >
> > > What happended since v6.6 ? On v6.7 I see
> > > [    2.978722] caam_jr 2142000.jr: 40000013: DECO: desc idx 0:
> > > Header Error. Invalid length or parity, or certain other problems.
> > >
> > > and later the above -EINVAL from the RSA verify callback, where I
> > > add
> > > the debug printing I see.
> > >
> > > What's the deal with this "leading 0 byte"?
> >
> > See RFC 2313, p 8.1
>
> hi Ignat,
>
> thanks for your time, the problem is *sometimes* rsa verify fails.
> there seems to be a race condition:

Can you clarify the failure case a bit? Is this the same signature
that fails? (That is, you just verify a fixed signature in a loop) Or
are these different signatures? (some reliably verify and some
reliably fail)

> in the failure-case after crypto_akcipher_encrypt() and
> crypto_wait_req() the *same* data as before is still at out_buf! that
> has not yet been written to.
>
> It's not that obvious to be yet because msleep(1000) doesn't change
> much and 00, 01, ff, ff... is *still* not yet written to out_buf!
>
> is there a reason why crypto_akcipher_sync_{en,de}crypt() is not used?
> Can you imagine what could go wrong here?
>
> *maybe* commit 1e562deacecca1f1bec7d23da526904a1e87525e that did a lot
> of things in parallel (in order to keep functionality similar) got
> something wrong?
>
> sidenote: when I use an elliptic curve key instead of rsa, everything
> works.
>
> also, the auto-free for child_req looks a bit dangerous when using
> out_buf, but ok :)
>
> maybe this rings a bell, I'll keep debugging,
>
>                             martin
>
>
> >
> > >
> > > thank you!
> > >
> > >                                     martin
> > >
> > >
> > >
> > >  crypto/rsa-pkcs1pad.c | 5 +++--
> > >  crypto/rsassa-pkcs1.c | 5 +++--
> > >  2 files changed, 6 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/crypto/rsa-pkcs1pad.c b/crypto/rsa-pkcs1pad.c
> > > index 50bdb18e7b483..65a4821e9758b 100644
> > > --- a/crypto/rsa-pkcs1pad.c
> > > +++ b/crypto/rsa-pkcs1pad.c
> > > @@ -191,9 +191,10 @@ static int pkcs1pad_decrypt_complete(struct
> > > akcipher_request *req, int err)
> > >
> > >         out_buf = req_ctx->out_buf;
> > >         if (dst_len == ctx->key_size) {
> > > -               if (out_buf[0] != 0x00)
> > > -                       /* Decrypted value had no leading 0 byte */
> > > +               if (out_buf[0] != 0x00) {
> > > +                       pr_debug("Decrypted value had no leading 0
> > > byte\n");
> > >                         goto done;
> > > +               }
> > >
> > >                 dst_len--;
> > >                 out_buf++;
> > > diff --git a/crypto/rsassa-pkcs1.c b/crypto/rsassa-pkcs1.c
> > > index 94fa5e9600e79..22919728ea1c8 100644
> > > --- a/crypto/rsassa-pkcs1.c
> > > +++ b/crypto/rsassa-pkcs1.c
> > > @@ -263,9 +263,10 @@ static int rsassa_pkcs1_verify(struct
> > > crypto_sig *tfm,
> > >                 return -EINVAL;
> > >
> > >         if (dst_len == ctx->key_size) {
> > > -               if (out_buf[0] != 0x00)
> > > -                       /* Encrypted value had no leading 0 byte */
> > > +               if (out_buf[0] != 0x00) {
> > > +                       pr_debug("Encrypted value had no leading 0
> > > byte\n");
> > >                         return -EINVAL;
> > > +               }
> > >
> > >                 dst_len--;
> > >                 out_buf++;
> > > --
> > > 2.47.3
> > >
> >
> > Ignat