[PATCH] kernel: sysctl: ignore invalid taint bits introduced via kernel.tainted and taint the kernel with TAINT_USER on writes

[Rafael Aquini]

The sysctl knob allows any user 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, as well as
it makes clear use of TAINT_USER flag to mark the kernel
tainted by user everytime a taint value is written
to the kernel.tainted sysctl.

Signed-off-by: Rafael Aquini <aqu...@redhat.com>
---
 kernel/sysctl.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 8a176d8727a3..f0a4fb38ac62 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -2623,17 +2623,32 @@ static int proc_taint(struct ctl_table *table, int 
write,
                return err;
 
        if (write) {
+               int i;
+
+               /*
+                * Ignore user input that would make us committing
+                * arbitrary invalid TAINT flags in the loop below.
+                */
+               tmptaint &= (1UL << TAINT_FLAGS_COUNT) - 1;
+
                /*
                 * 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);
                }
+
+               /*
+                * Users with SYS_ADMIN capability can include any arbitrary
+                * taint flag by writing to this interface. If that's the case,
+                * we also need to mark the kernel "tainted by user".
+                */
+               add_taint(TAINT_USER, LOCKDEP_STILL_OK);
        }
 
+
        return err;
 }
 
-- 
2.25.4