[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <Y7vvKHYXpe7KmwCI@gmail.com>
Date: Mon, 9 Jan 2023 11:40:40 +0100
From: Ingo Molnar <mingo@...nel.org>
To: "Maciej W. Rozycki" <macro@...am.me.uk>
Cc: Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>,
"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] x86: Disable kernel stack offset randomization for
!TSC
* Maciej W. Rozycki <macro@...am.me.uk> wrote:
> For x86 kernel stack offset randomization uses the RDTSC instruction,
> which causes an invalid opcode exception with hardware that does not
> implement this instruction:
> @@ -85,7 +86,8 @@ static inline void arch_exit_to_user_mod
> * Therefore, final stack offset entropy will be 5 (x86_64) or
> * 6 (ia32) bits.
> */
> - choose_random_kstack_offset(rdtsc() & 0xFF);
> + if (cpu_feature_enabled(X86_FEATURE_TSC))
> + choose_random_kstack_offset(rdtsc() & 0xFF);
> }
While this is an obscure corner case, falling back to 0 offset silently
feels a bit wrong - could we at least attempt to generate some
unpredictability in this case?
It's not genuine entropy, but we could pass in a value that varies from
task to task and which is not an 'obviously known' constant value like the
0 fallback?
For example the lowest 8 bits of the virtual page number of the current
task plus the lowest 8 bits of jiffies should vary from task to task, has
some time dependence and is cheap to compute:
(((unsigned long)current >> 12) + jiffies) & 0xFF
This combined with the per-CPU forward storage of previous offsets:
#define choose_random_kstack_offset(rand) do { \
if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
&randomize_kstack_offset)) { \
u32 offset = raw_cpu_read(kstack_offset); \
offset ^= (rand); \
raw_cpu_write(kstack_offset, offset); \
} \
Should make this reasonably hard to guess for long-running tasks even if
there's no TSC - and make it hard to guess even for tasks whose creation an
attacker controls, unless there's an info-leak to rely on.
Thanks,
Ingo
Powered by blists - more mailing lists