[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-devel] Fix for incorrect SYSRET instruction implementation --
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] Fix for incorrect SYSRET instruction implementation -- anyone looked at this yet? |
Date: |
Tue, 10 Mar 2015 10:38:47 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0 |
On 09/03/2015 23:48, Bill Paul wrote:
> I'm certain I'm sending this in plain text mode this time. According to my
> reading of the Intel documentation, the SYSRET instruction is supposed to
> force the RPL bits of the %ss register to 3 when returning to user mode. The
> actual sequence is:
>
> SS.Selector <-- (IA32_STAR[63:48]+8) OR 3; (* RPL forced to 3 *)
>
> However, the code in helper_sysret() leaves them at 0 (in other words, the
> "OR
> 3" part of the above sequence is missing). It does set the privilege level
> bits of %cs correctly though.
>
> This has caused me trouble with some of my VxWorks development: code that
> runs
> okay on real hardware will crash on QEMU, unless I apply the patch below.
>
> Can someone confirm that this is in fact a real bug? The Intel architecture
> manual seems quite clear about the SYSRET behavior. The bug seems to have
> been
> around as far back as QEMU 0.10.5.
>
> I am using QEMU 2.2.0 on FreeBSD/amd64 9.1-RELEASE.
>
> -Bill
>
> Signed-off-by: Bill Paul <address@hidden>
>
> ---
> target-i386/seg_helper.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/target-i386/seg_helper.c b/target-i386/seg_helper.c
> index fa374d0..2bc757a 100644
> --- a/target-i386/seg_helper.c
> +++ b/target-i386/seg_helper.c
> @@ -1043,7 +1043,7 @@ void helper_sysret(CPUX86State *env, int dflag)
> DESC_CS_MASK | DESC_R_MASK | DESC_A_MASK);
> env->eip = (uint32_t)env->regs[R_ECX];
> }
> - cpu_x86_load_seg_cache(env, R_SS, selector + 8,
> + cpu_x86_load_seg_cache(env, R_SS, (selector + 8) | 3,
> 0, 0xffffffff,
> DESC_G_MASK | DESC_B_MASK | DESC_P_MASK |
> DESC_S_MASK | (3 << DESC_DPL_SHIFT) |
> @@ -1056,7 +1056,7 @@ void helper_sysret(CPUX86State *env, int dflag)
> DESC_S_MASK | (3 << DESC_DPL_SHIFT) |
> DESC_CS_MASK | DESC_R_MASK | DESC_A_MASK);
> env->eip = (uint32_t)env->regs[R_ECX];
> - cpu_x86_load_seg_cache(env, R_SS, selector + 8,
> + cpu_x86_load_seg_cache(env, R_SS, (selector + 8) | 3,
> 0, 0xffffffff,
> DESC_G_MASK | DESC_B_MASK | DESC_P_MASK |
> DESC_S_MASK | (3 << DESC_DPL_SHIFT) |
>
Applied, thanks. I will send a pull request today.
Paolo