linux-kernel - [PATCH 2/3] ptrace, seccomp: tweak get

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Message-Id: <20180221024747.9659-3-tycho@tycho.ws>
Date:   Tue, 20 Feb 2018 19:47:46 -0700
From:   Tycho Andersen <tycho@...ho.ws>
To:     linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>
Cc:     Tycho Andersen <tycho@...ho.ws>, Oleg Nesterov <oleg@...hat.com>
Subject: [PATCH 2/3] ptrace, seccomp: tweak get_metadata behavior slightly

Previously if users passed a small size for the input structure size, they
would get get odd behavior. It doesn't make sense to pass a structure
smaller than at least filter_off size, so let's just give -EINVAL in this
case.

This changes userspace visible behavior, but was only introduced in commit
26500475ac1b ("ptrace, seccomp: add support for retrieving seccomp
metadata") in 4.16-rc2, so should be safe to change if merged before then.

Reported-by: Eugene Syromiatnikov <esyr@...hat.com>
Signed-off-by: Tycho Andersen <tycho@...ho.ws>
CC: Kees Cook <keescook@...omium.org>
CC: Oleg Nesterov <oleg@...hat.com>
---
 kernel/seccomp.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 940fa408a288..dc77548167ef 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -1076,14 +1076,16 @@ long seccomp_get_metadata(struct task_struct *task,
 
 	size = min_t(unsigned long, size, sizeof(kmd));
 
-	if (copy_from_user(&kmd, data, size))
+	if (size < sizeof(kmd.filter_off))
+		return -EINVAL;
+
+	if (copy_from_user(&kmd.filter_off, data, sizeof(kmd.filter_off)))
 		return -EFAULT;
 
 	filter = get_nth_filter(task, kmd.filter_off);
 	if (IS_ERR(filter))
 		return PTR_ERR(filter);
 
-	memset(&kmd, 0, sizeof(kmd));
 	if (filter->log)
 		kmd.flags |= SECCOMP_FILTER_FLAG_LOG;
 
-- 
2.14.1