Re: [Qemu-devel] [PATCH] target-i386: Fixed syscall posssible segfault

[Paolo Bonzini]

On 13/09/2016 15:23, Stanislav Shmarov wrote:
> In user-mode emulation env->idt.base memory is
> allocated in linux-user/main.c with
> size 8*512 = 4096 (for 64-bit).
> When fake interrupt EXCP_SYSCALL is thrown
> do_interrupt_user checks destination privilege level
> for this fake exception, and tries to read 4 bytes
> at address base + (256 * 2^4)=4096, that causes
> segfault.
> 
> Privlege level was checked only for int's, so lets
> read dpl from memory only for this case.
> 
> Signed-off-by: Stanislav Shmarov <address@hidden>

Queued for 2.8, thanks.

Paolo

> ---
>  target-i386/seg_helper.c | 36 +++++++++++++++++++-----------------
>  1 file changed, 19 insertions(+), 17 deletions(-)
> 
> diff --git a/target-i386/seg_helper.c b/target-i386/seg_helper.c
> index 6cbdf17..fb79f31 100644
> --- a/target-i386/seg_helper.c
> +++ b/target-i386/seg_helper.c
> @@ -1137,25 +1137,27 @@ static void do_interrupt_real(CPUX86State *env, int 
> intno, int is_int,
>  static void do_interrupt_user(CPUX86State *env, int intno, int is_int,
>                                int error_code, target_ulong next_eip)
>  {
> -    SegmentCache *dt;
> -    target_ulong ptr;
> -    int dpl, cpl, shift;
> -    uint32_t e2;
> +    if (is_int) {
> +        SegmentCache *dt;
> +        target_ulong ptr;
> +        int dpl, cpl, shift;
> +        uint32_t e2;
>  
> -    dt = &env->idt;
> -    if (env->hflags & HF_LMA_MASK) {
> -        shift = 4;
> -    } else {
> -        shift = 3;
> -    }
> -    ptr = dt->base + (intno << shift);
> -    e2 = cpu_ldl_kernel(env, ptr + 4);
> +        dt = &env->idt;
> +        if (env->hflags & HF_LMA_MASK) {
> +            shift = 4;
> +        } else {
> +            shift = 3;
> +        }
> +        ptr = dt->base + (intno << shift);
> +        e2 = cpu_ldl_kernel(env, ptr + 4);
>  
> -    dpl = (e2 >> DESC_DPL_SHIFT) & 3;
> -    cpl = env->hflags & HF_CPL_MASK;
> -    /* check privilege if software int */
> -    if (is_int && dpl < cpl) {
> -        raise_exception_err(env, EXCP0D_GPF, (intno << shift) + 2);
> +        dpl = (e2 >> DESC_DPL_SHIFT) & 3;
> +        cpl = env->hflags & HF_CPL_MASK;
> +        /* check privilege if software int */
> +        if (dpl < cpl) {
> +            raise_exception_err(env, EXCP0D_GPF, (intno << shift) + 2);
> +        }
>      }
>  
>      /* Since we emulate only user space, we cannot do more than
>