Using the MSR instruction to write to CPSR.E is deprecated, but it is
required to work from any mode including unprivileged code.  We were
incorrectly forbidding usermode code from writing it because
CPSR_USER did not include the CPSR_E bit.

We use CPSR_USER in only three places:
 * as the mask of what to allow userspace MSR to write to CPSR
 * when deciding what bits a linux-user signal-return should be
   able to write from the sigcontext structure
 * in target_user_copy_regs() when we set up the initial
   registers for the linux-user process

In the first two cases not being able to update CPSR.E is a bug, and
in the third case it doesn't matter because CPSR.E is always 0 there.
So we can fix both bugs by adding CPSR_E to CPSR_USER.

Because the cpsr_write() in restore_sigcontext() is now changing
a CPSR bit which is cached in hflags, we need to add an
arm_rebuild_hflags() call there; the callsite in
target_user_copy_regs() was already rebuilding hflags for other
reasons.

(The recommended way to change CPSR.E is to use the 'SETEND'
instruction, which we do correctly allow from usermode code.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
v2 changes:
 * fixed wrong variable name in commit message
 * added arm_rebuild_hflags() call in restore_sigcontext()
---
 target/arm/cpu.h        | 2 +-
 linux-user/arm/signal.c | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/target/arm/cpu.h b/target/arm/cpu.h
index 5d995368d4f..677584e5da0 100644
--- a/target/arm/cpu.h
+++ b/target/arm/cpu.h
@@ -1230,7 +1230,7 @@ void pmu_init(ARMCPU *cpu);
 #define CACHED_CPSR_BITS (CPSR_T | CPSR_AIF | CPSR_GE | CPSR_IT | CPSR_Q \
     | CPSR_NZCV)
 /* Bits writable in user mode.  */
-#define CPSR_USER (CPSR_NZCV | CPSR_Q | CPSR_GE)
+#define CPSR_USER (CPSR_NZCV | CPSR_Q | CPSR_GE | CPSR_E)
 /* Execution state bits.  MRS read as zero, MSR writes ignored.  */
 #define CPSR_EXEC (CPSR_T | CPSR_IT | CPSR_J | CPSR_IL)
 
diff --git a/linux-user/arm/signal.c b/linux-user/arm/signal.c
index a475a103e97..698985a647e 100644
--- a/linux-user/arm/signal.c
+++ b/linux-user/arm/signal.c
@@ -552,6 +552,7 @@ restore_sigcontext(CPUARMState *env, struct target_sigcontext *sc)
 #ifdef TARGET_CONFIG_CPU_32
     __get_user(cpsr, &sc->arm_cpsr);
     cpsr_write(env, cpsr, CPSR_USER | CPSR_EXEC, CPSRWriteByInstr);
+    arm_rebuild_hflags(env);
 #endif
 
     err |= !valid_user_regs(env);
-- 
2.20.1

On 5/18/20 7:28 AM, Peter Maydell wrote:
> Using the MSR instruction to write to CPSR.E is deprecated, but it is
> required to work from any mode including unprivileged code.  We were
> incorrectly forbidding usermode code from writing it because
> CPSR_USER did not include the CPSR_E bit.
> 
> We use CPSR_USER in only three places:
>  * as the mask of what to allow userspace MSR to write to CPSR
>  * when deciding what bits a linux-user signal-return should be
>    able to write from the sigcontext structure
>  * in target_user_copy_regs() when we set up the initial
>    registers for the linux-user process
> 
> In the first two cases not being able to update CPSR.E is a bug, and
> in the third case it doesn't matter because CPSR.E is always 0 there.
> So we can fix both bugs by adding CPSR_E to CPSR_USER.
> 
> Because the cpsr_write() in restore_sigcontext() is now changing
> a CPSR bit which is cached in hflags, we need to add an
> arm_rebuild_hflags() call there; the callsite in
> target_user_copy_regs() was already rebuilding hflags for other
> reasons.
> 
> (The recommended way to change CPSR.E is to use the 'SETEND'
> instruction, which we do correctly allow from usermode code.)
> 
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
> v2 changes:
>  * fixed wrong variable name in commit message
>  * added arm_rebuild_hflags() call in restore_sigcontext()
> ---
>  target/arm/cpu.h        | 2 +-
>  linux-user/arm/signal.c | 1 +
>  2 files changed, 2 insertions(+), 1 deletion(-)

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>


r~

On Mon, 18 May 2020 at 15:28, Peter Maydell <peter.maydell@linaro.org> wrote:
>
> Using the MSR instruction to write to CPSR.E is deprecated, but it is
> required to work from any mode including unprivileged code.  We were
> incorrectly forbidding usermode code from writing it because
> CPSR_USER did not include the CPSR_E bit.
>
> We use CPSR_USER in only three places:
>  * as the mask of what to allow userspace MSR to write to CPSR
>  * when deciding what bits a linux-user signal-return should be
>    able to write from the sigcontext structure
>  * in target_user_copy_regs() when we set up the initial
>    registers for the linux-user process
>
> In the first two cases not being able to update CPSR.E is a bug, and
> in the third case it doesn't matter because CPSR.E is always 0 there.
> So we can fix both bugs by adding CPSR_E to CPSR_USER.
>
> Because the cpsr_write() in restore_sigcontext() is now changing
> a CPSR bit which is cached in hflags, we need to add an
> arm_rebuild_hflags() call there; the callsite in
> target_user_copy_regs() was already rebuilding hflags for other
> reasons.
>
> (The recommended way to change CPSR.E is to use the 'SETEND'
> instruction, which we do correctly allow from usermode code.)
>
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>



Applied to target-arm.next, thanks.

-- PMM