Message-ID: <63dc2059.170a0220.25fcc.0394@mx.google.com>
Date:   Thu, 2 Feb 2023 20:43:04 +0000
From:   Kees Cook <keescook@chromium.org>
To:     Sami Tolvanen <samitolvanen@google.com>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Ard Biesheuvel <ardb@kernel.org>,
        John Stultz <jstultz@google.com>,
        Yongqin Liu <yongqin.liu@linaro.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Rasmus Villemoes <rasmus.villemoes@prevas.dk>,
        Thomas Gleixner <tglx@linutronix.de>,
        Jason Gunthorpe <jgg@ziepe.ca>, Nishanth Menon <nm@ti.com>,
        Michael Kelley <mikelley@microsoft.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Won Chung <wonchung@google.com>,
        David Gow <davidgow@google.com>,
        Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Hyeonggon Yoo <42.hyeyoo@gmail.com>,
        Guenter Roeck <linux@roeck-us.net>,
        Andy Shevchenko <andriy.shevchenko@intel.com>,
        Paolo Abeni <pabeni@redhat.com>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Tom Rix <trix@redhat.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-hardening@vger.kernel.org,
        llvm@lists.linux.dev, Sumit Semwal <sumit.semwal@linaro.org>
Subject: Re: [PATCH 5/6] driver core: Add __alloc_size hint to devm allocators
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On Thu, Feb 02, 2023 at 12:11:47PM -0800, Sami Tolvanen wrote:
> On Thu, Feb 2, 2023 at 11:53 AM Kees Cook <keescook@chromium.org> wrote:
> >
> > On Thu, Feb 02, 2023 at 11:49:42AM -0800, Sami Tolvanen wrote:
> > > A quick look at Clang's source code suggests that Intrinsic::ubsantrap
> > > already accepts the handler ID (from the SanitizerHandler enum) as an
> > > argument and the arm64 LLVM back-end appears to encode the value as an
> > > immediate for the brk instruction. I didn't confirm that this actually
> > > works, but perhaps we just need to teach the kernel about the possible
> > > values?
> >
> > Oh excellent. Yeah, if that's all that's needed here that would be
> > great. What are the values?
> 
> The arm64 brk immediate encoding seems to be "ubsantrap arg | 'U' << 8":
> 
> https://github.com/llvm/llvm-project/blob/main/llvm/lib/Target/AArch64/AArch64InstrInfo.td#L7571
> 
> The argument values come from the SanitizerHandler enum, which is
> populated from this list:
> 
> https://github.com/llvm/llvm-project/blob/main/clang/lib/CodeGen/CodeGenFunction.h#L113
> 
> Therefore, according to the tests, for ubsantrap(12) we'll get brk
> #0x550c, for example:
> 
> https://github.com/llvm/llvm-project/blob/main/llvm/test/CodeGen/AArch64/ubsantrap.ll

So the absolute minimal handler would look like this:

diff --git a/arch/arm64/include/asm/brk-imm.h b/arch/arm64/include/asm/brk-imm.h
index 6e000113e508..3f0f0d03268b 100644
--- a/arch/arm64/include/asm/brk-imm.h
+++ b/arch/arm64/include/asm/brk-imm.h
@@ -28,6 +28,8 @@
 #define BUG_BRK_IMM			0x800
 #define KASAN_BRK_IMM			0x900
 #define KASAN_BRK_MASK			0x0ff
+#define UBSAN_BRK_IMM			0x5500
+#define UBSAN_BRK_MASK			0x00ff
 
 #define CFI_BRK_IMM_TARGET		GENMASK(4, 0)
 #define CFI_BRK_IMM_TYPE		GENMASK(9, 5)
diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
index 4c0caa589e12..36b917d8fa5f 100644
--- a/arch/arm64/kernel/traps.c
+++ b/arch/arm64/kernel/traps.c
@@ -1074,6 +1074,18 @@ static struct break_hook kasan_break_hook = {
 };
 #endif
 
+#ifdef CONFIG_UBSAN_TRAP
+static int ubsan_handler(struct pt_regs *regs, unsigned long esr)
+{
+	die("Oops - UBSAN", regs, esr);
+}
+
+static struct break_hook ubsan_break_hook = {
+	.fn	= ubsan_handler,
+	.imm	= UBSAN_BRK_IMM,
+	.mask	= UBSAN_BRK_MASK,
+};
+#endif
 
 #define esr_comment(esr) ((esr) & ESR_ELx_BRK64_ISS_COMMENT_MASK)
 
@@ -1091,6 +1103,10 @@ int __init early_brk64(unsigned long addr, unsigned long esr,
 #ifdef CONFIG_KASAN_SW_TAGS
 	if ((esr_comment(esr) & ~KASAN_BRK_MASK) == KASAN_BRK_IMM)
 		return kasan_handler(regs, esr) != DBG_HOOK_HANDLED;
+#endif
+#ifdef CONFIG_UBSAN_TRAP
+	if ((esr_comment(esr) & ~UBSAN_BRK_MASK) == UBSAN_BRK_IMM)
+		return ubsan_handler(regs, esr) != DBG_HOOK_HANDLED;
 #endif
 	return bug_handler(regs, esr) != DBG_HOOK_HANDLED;
 }
@@ -1104,6 +1120,9 @@ void __init trap_init(void)
 	register_kernel_break_hook(&fault_break_hook);
 #ifdef CONFIG_KASAN_SW_TAGS
 	register_kernel_break_hook(&kasan_break_hook);
+#endif
+#ifdef CONFIG_UBSAN_TRAP
+	register_kernel_break_hook(&ubsan_break_hook);
 #endif
 	debug_traps_init();
 }

But we could expand ubsan_handler() to extract the SanitizerHandler enum
value and report which UBSAN check was hit...

-- 
Kees Cook