linux-kernel - Re: [PATCH v8 05/28] x86/mpx: Use signed variables to compute effective addresses

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Message-ID: <20170829160906.gpenny6yqq5squ6l@pd.tnic>
Date:   Tue, 29 Aug 2017 18:09:06 +0200
From:   Borislav Petkov <bp@...e.de>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Brian Gerst <brgerst@...il.com>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Liang Z Li <liang.z.li@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Huang Rui <ray.huang@....com>, Jiri Slaby <jslaby@...e.cz>,
        Jonathan Corbet <corbet@....net>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Chen Yucong <slaoub@...il.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org,
        x86@...nel.org, ricardo.neri@...el.com,
        Adam Buchbinder <adam.buchbinder@...il.com>,
        Colin Ian King <colin.king@...onical.com>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Qiaowei Ren <qiaowei.ren@...el.com>,
        Nathan Howard <liverlint@...il.com>,
        Adan Hawthorn <adanhawthorn@...il.com>,
        Joe Perches <joe@...ches.com>
Subject: Re: [PATCH v8 05/28] x86/mpx: Use signed variables to compute
 effective addresses

On Fri, Aug 18, 2017 at 05:27:46PM -0700, Ricardo Neri wrote:
> Even though memory addresses are unsigned, the operands used to compute the
> effective address do have a sign. This is true for ModRM.rm, SIB.base,
> SIB.index as well as the displacement bytes. Thus, signed variables shall
> be used when computing the effective address from these operands. Once the
> signed effective address has been computed, it is casted to an unsigned
> long to determine the linear address.
> 
> Variables are renamed to better reflect the type of address being
> computed.
> 
> Cc: Borislav Petkov <bp@...e.de>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Adam Buchbinder <adam.buchbinder@...il.com>
> Cc: Colin Ian King <colin.king@...onical.com>
> Cc: Lorenzo Stoakes <lstoakes@...il.com>
> Cc: Qiaowei Ren <qiaowei.ren@...el.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Nathan Howard <liverlint@...il.com>
> Cc: Adan Hawthorn <adanhawthorn@...il.com>
> Cc: Joe Perches <joe@...ches.com>
> Cc: Ravi V. Shankar <ravi.v.shankar@...el.com>
> Cc: x86@...nel.org
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
> ---
>  arch/x86/mm/mpx.c | 20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)

I think you can simplify this function even more (diff ontop):

diff --git a/arch/x86/mm/mpx.c b/arch/x86/mm/mpx.c
index 9eec98022510..d0ec5c9b2a57 100644
--- a/arch/x86/mm/mpx.c
+++ b/arch/x86/mm/mpx.c
@@ -139,7 +139,7 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs,
 static void __user *mpx_get_addr_ref(struct insn *insn, struct pt_regs *regs)
 {
 	int addr_offset, base_offset, indx_offset;
-	unsigned long linear_addr;
+	unsigned long linear_addr = -1;
 	long eff_addr, base, indx;
 	insn_byte_t sib;
 
@@ -150,18 +150,18 @@ static void __user *mpx_get_addr_ref(struct insn *insn, struct pt_regs *regs)
 	if (X86_MODRM_MOD(insn->modrm.value) == 3) {
 		addr_offset = get_reg_offset(insn, regs, REG_TYPE_RM);
 		if (addr_offset < 0)
-			goto out_err;
+			goto out;
 
 		eff_addr = regs_get_register(regs, addr_offset);
 	} else {
 		if (insn->sib.nbytes) {
 			base_offset = get_reg_offset(insn, regs, REG_TYPE_BASE);
 			if (base_offset < 0)
-				goto out_err;
+				goto out;
 
 			indx_offset = get_reg_offset(insn, regs, REG_TYPE_INDEX);
 			if (indx_offset < 0)
-				goto out_err;
+				goto out;
 
 			base = regs_get_register(regs, base_offset);
 			indx = regs_get_register(regs, indx_offset);
@@ -170,7 +170,7 @@ static void __user *mpx_get_addr_ref(struct insn *insn, struct pt_regs *regs)
 		} else {
 			addr_offset = get_reg_offset(insn, regs, REG_TYPE_RM);
 			if (addr_offset < 0)
-				goto out_err;
+				goto out;
 
 			eff_addr = regs_get_register(regs, addr_offset);
 		}
@@ -180,9 +180,8 @@ static void __user *mpx_get_addr_ref(struct insn *insn, struct pt_regs *regs)
 
 	linear_addr = (unsigned long)eff_addr;
 
+out:
 	return (void __user *)linear_addr;
-out_err:
-	return (void __user *)-1;
 }
 
 static int mpx_insn_decode(struct insn *insn,

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--