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Message-ID: <20231209155308.GAZXSNZB9PPlcHBI4W@fat_crate.local>
Date: Sat, 9 Dec 2023 16:53:08 +0100
From: Borislav Petkov <bp@...en8.de>
To: Michal Luczaj <mhal@...x.co>
Cc: x86@...nel.org, tglx@...utronix.de, mingo@...hat.com,
dave.hansen@...ux.intel.com, shuah@...nel.org, luto@...nel.org,
torvalds@...uxfoundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] x86: UMIP emulation leaking kernel addresses
On Wed, Dec 06, 2023 at 01:43:43AM +0100, Michal Luczaj wrote:
> Introducing a DPL check in insn_get_seg_base(), or even in get_desc(),
> seems enough to prevent the decoder from disclosing data.
>
> diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
> index 558a605929db..4c1eea736519 100644
> --- a/arch/x86/lib/insn-eval.c
> +++ b/arch/x86/lib/insn-eval.c
> @@ -725,6 +725,18 @@ unsigned long insn_get_seg_base(struct pt_regs *regs, int seg_reg_idx)
> if (!get_desc(&desc, sel))
> return -1L;
>
> + /*
> + * Some segment selectors coming from @regs do not necessarily reflect
> + * the state of CPU; see get_segment_selector(). Their values might
> + * have been altered by ptrace. Thus, the instruction decoder can be
> + * tricked into "dereferencing" a segment descriptor that would
> + * otherwise cause a CPU exception -- for example due to mismatched
> + * privilege levels. This opens up the possibility to expose kernel
> + * space base address of DPL=0 segments.
> + */
> + if (desc.dpl < (regs->cs & SEGMENT_RPL_MASK))
> + return -1L;
> +
> return get_desc_base(&desc);
> }
>
> That said, I guess instead of trying to harden the decoder,
Well, here's what my CPU manual says:
"4.10.1 Accessing Data Segments
...
The processor compares the effective privilege level with the DPL in the
descriptor-table entry referenced by the segment selector. If the
effective privilege level is greater than or equal to (numerically
lower-than or equal-to) the DPL, then the processor loads the segment
register with the data-segment selector.
If the effective privilege level is lower than (numerically
greater-than) the DPL, a general-protection exception (#GP) occurs and
the segment register is not loaded.
...
4.10.2 Accessing Stack Segments
The processor compares the CPL with the DPL in the descriptor-table
entry referenced by the segment selector. The two values must be equal.
If they are not equal, a #GP occurs and the SS register is not loaded."
So *actually* doing those checks in the insn decoder is the proper thing
to do, IMNSVHO.
> Now, I'm far from being competent, but here's an idea I've tried: tell
> the #GP handler that UMIP-related exceptions come only as #GP(0):
>
> if (static_cpu_has(X86_FEATURE_UMIP)) {
> - if (user_mode(regs) && fixup_umip_exception(regs))
> + if (user_mode(regs) && !error_code && fixup_umip_exception(regs))
> goto exit;
> }
And yap, as you've realized, that alone doesn't fix the leaking.
Thx.
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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