linux-kernel - [PATCH] kernel: sysctl: ignore out-of-range taint bits introduced via kernel.tainted

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Date:   Tue, 12 May 2020 13:46:53 -0400
From:   Rafael Aquini <aquini@...hat.com>
To:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        mcgrof@...nel.org
Cc:     keescook@...omium.org, akpm@...ux-foundation.org,
        yzaikin@...gle.com, tytso@....edu
Subject: [PATCH] kernel: sysctl: ignore out-of-range taint bits introduced via kernel.tainted

The sysctl knob allows users with SYS_ADMIN capability to
taint the kernel with any arbitrary value, but this might
produce an invalid flags bitset being committed to tainted_mask.

This patch introduces a simple way for proc_taint() to ignore
any eventual invalid bit coming from the user input before
committing those bits to the kernel tainted_mask.

Signed-off-by: Rafael Aquini <aquini@...hat.com>
---
 include/linux/kernel.h |  2 ++
 kernel/sysctl.c        | 14 +++++++++++++-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 9b7a8d74a9d6..e8c22a9bbc95 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -597,6 +597,8 @@ extern enum system_states {
 #define TAINT_RANDSTRUCT		17
 #define TAINT_FLAGS_COUNT		18
 
+#define TAINT_FLAGS_MAX			((1UL << TAINT_FLAGS_COUNT) - 1)
+
 struct taint_flag {
 	char c_true;	/* character printed when tainted */
 	char c_false;	/* character printed when not tainted */
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 8a176d8727a3..fb2d693fc08c 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -2623,11 +2623,23 @@ static int proc_taint(struct ctl_table *table, int write,
 		return err;
 
 	if (write) {
+		int i;
+
+		/*
+		 * Ignore user input that would cause the loop below
+		 * to commit arbitrary and out of valid range TAINT flags.
+		 */
+		if (tmptaint > TAINT_FLAGS_MAX) {
+			tmptaint &= TAINT_FLAGS_MAX;
+			pr_warn_once("%s: out-of-range taint input ignored."
+				     " tainted_mask adjusted to 0x%lx\n",
+				     __func__, tmptaint);
+		}
+
 		/*
 		 * Poor man's atomic or. Not worth adding a primitive
 		 * to everyone's atomic.h for this
 		 */
-		int i;
 		for (i = 0; i < BITS_PER_LONG && tmptaint >> i; i++) {
 			if ((tmptaint >> i) & 1)
 				add_taint(i, LOCKDEP_STILL_OK);
-- 
2.25.4