| Message ID | 20230802041503.11530-1-ebiggers@kernel.org (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | [v2] fsverity: skip PKCS#7 parser when keyring is empty | expand |
On Wed Aug 2, 2023 at 7:15 AM EEST, Eric Biggers wrote: > From: Eric Biggers <ebiggers@google.com> > > If an fsverity builtin signature is given for a file but the > ".fs-verity" keyring is empty, there's no real reason to run the PKCS#7 > parser. Skip this to avoid the PKCS#7 attack surface when builtin > signature support is configured into the kernel but is not being used. > > This is a hardening improvement, not a fix per se, but I've added > Fixes and Cc stable to get it out to more users. > > Fixes: 432434c9f8e1 ("fs-verity: support builtin file signatures") > Cc: stable@vger.kernel.org > Acked-by: Jarkko Sakkinen <jarkko@kernel.org> > Signed-off-by: Eric Biggers <ebiggers@google.com> > --- > > v2: check keyring and return early before allocating formatted digest > > fs/verity/signature.c | 15 +++++++++++++++ > 1 file changed, 15 insertions(+) > > diff --git a/fs/verity/signature.c b/fs/verity/signature.c > index b95acae64eac6..8f474702aa249 100644 > --- a/fs/verity/signature.c > +++ b/fs/verity/signature.c > @@ -62,6 +62,21 @@ int fsverity_verify_signature(const struct fsverity_info *vi, > return 0; > } > > + if (fsverity_keyring->keys.nr_leaves_on_tree == 0) { > + /* > + * The ".fs-verity" keyring is empty, due to builtin signatures > + * being supported by the kernel but not actually being used. > + * In this case, verify_pkcs7_signature() would always return an > + * error, usually ENOKEY. It could also be EBADMSG if the > + * PKCS#7 is malformed, but that isn't very important to > + * distinguish. So, just skip to ENOKEY to avoid the attack > + * surface of the PKCS#7 parser, which would otherwise be > + * reachable by any task able to execute FS_IOC_ENABLE_VERITY. > + */ > + fsverity_err(inode, "fs-verity keyring is empty"); > + return -ENOKEY; > + } > + > d = kzalloc(sizeof(*d) + hash_alg->digest_size, GFP_KERNEL); > if (!d) > return -ENOMEM; > > base-commit: 456ae5fe9b448f44ebe98b391a3bae9c75df465e > -- > 2.41.0 Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org> applied BR, Jarkko
On Wed, Aug 02, 2023 at 07:27:15AM +0300, Jarkko Sakkinen wrote: > On Wed Aug 2, 2023 at 7:15 AM EEST, Eric Biggers wrote: > > From: Eric Biggers <ebiggers@google.com> > > > > If an fsverity builtin signature is given for a file but the > > ".fs-verity" keyring is empty, there's no real reason to run the PKCS#7 > > parser. Skip this to avoid the PKCS#7 attack surface when builtin > > signature support is configured into the kernel but is not being used. > > > > This is a hardening improvement, not a fix per se, but I've added > > Fixes and Cc stable to get it out to more users. > > > > Fixes: 432434c9f8e1 ("fs-verity: support builtin file signatures") > > Cc: stable@vger.kernel.org > > Acked-by: Jarkko Sakkinen <jarkko@kernel.org> > > Signed-off-by: Eric Biggers <ebiggers@google.com> > > --- > > > > v2: check keyring and return early before allocating formatted digest > > > > fs/verity/signature.c | 15 +++++++++++++++ > > 1 file changed, 15 insertions(+) > > > > diff --git a/fs/verity/signature.c b/fs/verity/signature.c > > index b95acae64eac6..8f474702aa249 100644 > > --- a/fs/verity/signature.c > > +++ b/fs/verity/signature.c > > @@ -62,6 +62,21 @@ int fsverity_verify_signature(const struct fsverity_info *vi, > > return 0; > > } > > > > + if (fsverity_keyring->keys.nr_leaves_on_tree == 0) { > > + /* > > + * The ".fs-verity" keyring is empty, due to builtin signatures > > + * being supported by the kernel but not actually being used. > > + * In this case, verify_pkcs7_signature() would always return an > > + * error, usually ENOKEY. It could also be EBADMSG if the > > + * PKCS#7 is malformed, but that isn't very important to > > + * distinguish. So, just skip to ENOKEY to avoid the attack > > + * surface of the PKCS#7 parser, which would otherwise be > > + * reachable by any task able to execute FS_IOC_ENABLE_VERITY. > > + */ > > + fsverity_err(inode, "fs-verity keyring is empty"); > > + return -ENOKEY; > > + } > > + > > d = kzalloc(sizeof(*d) + hash_alg->digest_size, GFP_KERNEL); > > if (!d) > > return -ENOMEM; > > > > base-commit: 456ae5fe9b448f44ebe98b391a3bae9c75df465e > > -- > > 2.41.0 > > Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org> > > applied > > BR, Jarkko Hi Jarkko, thanks for the review! I actually intended to take this through the fsverity tree. Is that okay? BTW, we could actually make this change to verify_pkcs7_signature() itself. I wasn't sure it would be appropriate for all callers, though. Any thoughts? - Eric
On Wed Aug 2, 2023 at 7:31 AM EEST, Eric Biggers wrote: > On Wed, Aug 02, 2023 at 07:27:15AM +0300, Jarkko Sakkinen wrote: > > On Wed Aug 2, 2023 at 7:15 AM EEST, Eric Biggers wrote: > > > From: Eric Biggers <ebiggers@google.com> > > > > > > If an fsverity builtin signature is given for a file but the > > > ".fs-verity" keyring is empty, there's no real reason to run the PKCS#7 > > > parser. Skip this to avoid the PKCS#7 attack surface when builtin > > > signature support is configured into the kernel but is not being used. > > > > > > This is a hardening improvement, not a fix per se, but I've added > > > Fixes and Cc stable to get it out to more users. > > > > > > Fixes: 432434c9f8e1 ("fs-verity: support builtin file signatures") > > > Cc: stable@vger.kernel.org > > > Acked-by: Jarkko Sakkinen <jarkko@kernel.org> > > > Signed-off-by: Eric Biggers <ebiggers@google.com> > > > --- > > > > > > v2: check keyring and return early before allocating formatted digest > > > > > > fs/verity/signature.c | 15 +++++++++++++++ > > > 1 file changed, 15 insertions(+) > > > > > > diff --git a/fs/verity/signature.c b/fs/verity/signature.c > > > index b95acae64eac6..8f474702aa249 100644 > > > --- a/fs/verity/signature.c > > > +++ b/fs/verity/signature.c > > > @@ -62,6 +62,21 @@ int fsverity_verify_signature(const struct fsverity_info *vi, > > > return 0; > > > } > > > > > > + if (fsverity_keyring->keys.nr_leaves_on_tree == 0) { > > > + /* > > > + * The ".fs-verity" keyring is empty, due to builtin signatures > > > + * being supported by the kernel but not actually being used. > > > + * In this case, verify_pkcs7_signature() would always return an > > > + * error, usually ENOKEY. It could also be EBADMSG if the > > > + * PKCS#7 is malformed, but that isn't very important to > > > + * distinguish. So, just skip to ENOKEY to avoid the attack > > > + * surface of the PKCS#7 parser, which would otherwise be > > > + * reachable by any task able to execute FS_IOC_ENABLE_VERITY. > > > + */ > > > + fsverity_err(inode, "fs-verity keyring is empty"); > > > + return -ENOKEY; > > > + } > > > + > > > d = kzalloc(sizeof(*d) + hash_alg->digest_size, GFP_KERNEL); > > > if (!d) > > > return -ENOMEM; > > > > > > base-commit: 456ae5fe9b448f44ebe98b391a3bae9c75df465e > > > -- > > > 2.41.0 > > > > Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org> > > > > applied > > > > BR, Jarkko > > Hi Jarkko, thanks for the review! > > I actually intended to take this through the fsverity tree. Is that okay? > > BTW, we could actually make this change to verify_pkcs7_signature() itself. > I wasn't sure it would be appropriate for all callers, though. Any thoughts? It is OK for me. I just wanted make sure that I don't get yelled let's say month from now, why I haven't picked it already. That's why the "more eager" approach :-) I'll drop it from my master branch today. BR, Jarkko
diff --git a/fs/verity/signature.c b/fs/verity/signature.c index b95acae64eac6..8f474702aa249 100644 --- a/fs/verity/signature.c +++ b/fs/verity/signature.c @@ -62,6 +62,21 @@ int fsverity_verify_signature(const struct fsverity_info *vi, return 0; } + if (fsverity_keyring->keys.nr_leaves_on_tree == 0) { + /* + * The ".fs-verity" keyring is empty, due to builtin signatures + * being supported by the kernel but not actually being used. + * In this case, verify_pkcs7_signature() would always return an + * error, usually ENOKEY. It could also be EBADMSG if the + * PKCS#7 is malformed, but that isn't very important to + * distinguish. So, just skip to ENOKEY to avoid the attack + * surface of the PKCS#7 parser, which would otherwise be + * reachable by any task able to execute FS_IOC_ENABLE_VERITY. + */ + fsverity_err(inode, "fs-verity keyring is empty"); + return -ENOKEY; + } + d = kzalloc(sizeof(*d) + hash_alg->digest_size, GFP_KERNEL); if (!d) return -ENOMEM;