Date:   Tue, 9 Nov 2021 23:02:48 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Qing Zhao <qing.zhao@...cle.com>
Cc:     "ramana.radhakrishnan@....com" <ramana.radhakrishnan@....com>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>,
        kees Cook <keescook@...omium.org>,
        Keith Packard <keithpac@...zon.com>,
        "thomas.preudhomme@...est.fr" <thomas.preudhomme@...est.fr>,
        "adhemerval.zanella@...aro.org" <adhemerval.zanella@...aro.org>,
        Richard Sandiford <richard.sandiford@....com>,
        "gcc-patches@....gnu.org" <gcc-patches@....gnu.org>
Subject: Re: [PATCH v4 1/1] [ARM] Add support for TLS register based stack
 protector canary access

On Tue, 9 Nov 2021 at 21:45, Qing Zhao <qing.zhao@...cle.com> wrote:
>
> Hi, Ard,
>
> Sorry for the late reply (since I don’t have the right to approve a patch, I has been waiting for any arm port maintainer to review this patch).
> The following is the arm port maintainer information I got from MAINTAINERS file (you might want to explicitly cc’ing one of them for a review)
>
> arm port                Nick Clifton            <nickc@...hat.com>
> arm port                Richard Earnshaw        <richard.earnshaw@....com>
> arm port                Ramana Radhakrishnan    <ramana.radhakrishnan@....com>
> arm port                Kyrylo Tkachov          <kyrylo.tkachov@....com>
>
> I see that Ramana implemented the similar patch for aarch64 (commit cd0b2d361df82c848dc7e1c3078651bb0624c3c6), So, I am CCing him with this email. Hopefully he will review this patch.
>

Thank you Qing. But I know Ramana well, and I know he no longer works
on GCC. I collaborated with him on the AArch64 implementation at the
time (but he wrote all the code)

> Anyway, I briefly read your patch (version 4), and have the following questions and comments:
>
> 1.  When the option -mstack-protector-guard=tls presents,  should the option mstack-protector-guard-offset=.. be required to present?
>      If it’s required to present, you might want to add such requirement to the documentation, and also issue errors when it’s not present.
>      It’s not clear right now from the current implementation, so, you might need to update both "arm_option_override_internal “ in arm.c
>      and doc/invoke.texi to make this clear.
>

An  offset of 0x0 is a reasonable default, so I don't think it is
necessary to require the offset param to be passed in that case.

> 2. For arm, is there only one system register can be used for this purpose?
>

There are other registers that might be used in the same way, but the
TLS register is the obvious choice. On AArch64, we decided to use
'sysreg' and permit the user to specify the register because the Linux
kernel uses the user space stack pointer (SP_EL0), which is kind of
odd so we did not want to hard code that.

> 3. For the functionality you added, I didn’t see any testing cases added, I think testing cases are needed.
>

Yes, I am aware of that. I'm just not sure I know how to proceed here:
any pointers?

> More comments are embedded below:
>
> > On Oct 28, 2021, at 6:27 AM, Ard Biesheuvel <ardb@...nel.org> wrote:
> >
> > Add support for accessing the stack canary value via the TLS register,
> > so that multiple threads running in the same address space can use
> > distinct canary values. This is intended for the Linux kernel running in
> > SMP mode, where processes entering the kernel are essentially threads
> > running the same program concurrently: using a global variable for the
> > canary in that context is problematic because it can never be rotated,
> > and so the OS is forced to use the same value as long as it remains up.
> >
> > Using the TLS register to index the stack canary helps with this, as it
> > allows each CPU to context switch the TLS register along with the rest
> > of the process, permitting each process to use its own value for the
> > stack canary.
> >
> > 2021-10-28 Ard Biesheuvel <ardb@...nel.org>
> >
> >       * config/arm/arm-opts.h (enum stack_protector_guard): New
> >       * config/arm/arm-protos.h (arm_stack_protect_tls_canary_mem):
> >       New
> >       * config/arm/arm.c (TARGET_STACK_PROTECT_GUARD): Define
> >       (arm_option_override_internal): Handle and put in error checks
> >       for stack protector guard options.
> >       (arm_option_reconfigure_globals): Likewise
> >       (arm_stack_protect_tls_canary_mem): New
> >       (arm_stack_protect_guard): New
> >       * config/arm/arm.md (stack_protect_set): New
> >       (stack_protect_set_tls): Likewise
> >       (stack_protect_test): Likewise
> >       (stack_protect_test_tls): Likewise
> >       (reload_tp_hard): Likewise
> >       * config/arm/arm.opt (-mstack-protector-guard): New
> >       (-mstack-protector-guard-offset): New.
> >       * doc/invoke.texi: Document new options
> >
> > Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> > ---
> > gcc/config/arm/arm-opts.h   |  6 ++
> > gcc/config/arm/arm-protos.h |  2 +
> > gcc/config/arm/arm.c        | 55 +++++++++++++++
> > gcc/config/arm/arm.md       | 71 +++++++++++++++++++-
> > gcc/config/arm/arm.opt      | 22 ++++++
> > gcc/doc/invoke.texi         |  9 +++
> > 6 files changed, 163 insertions(+), 2 deletions(-)
> >
> > diff --git a/gcc/config/arm/arm-opts.h b/gcc/config/arm/arm-opts.h
> > index 5c4b62f404f7..581ba3c4fbbb 100644
> > --- a/gcc/config/arm/arm-opts.h
> > +++ b/gcc/config/arm/arm-opts.h
> > @@ -69,4 +69,10 @@ enum arm_tls_type {
> >   TLS_GNU,
> >   TLS_GNU2
> > };
> > +
> > +/* Where to get the canary for the stack protector.  */
> > +enum stack_protector_guard {
> > +  SSP_TLSREG,                  /* per-thread canary in TLS register */
> > +  SSP_GLOBAL                   /* global canary */
> > +};
> > #endif
> > diff --git a/gcc/config/arm/arm-protos.h b/gcc/config/arm/arm-protos.h
> > index 9b1f61394ad7..d8d605920c97 100644
> > --- a/gcc/config/arm/arm-protos.h
> > +++ b/gcc/config/arm/arm-protos.h
> > @@ -195,6 +195,8 @@ extern void arm_split_atomic_op (enum rtx_code, rtx, rtx, rtx, rtx, rtx, rtx);
> > extern rtx arm_load_tp (rtx);
> > extern bool arm_coproc_builtin_available (enum unspecv);
> > extern bool arm_coproc_ldc_stc_legitimate_address (rtx);
> > +extern rtx arm_stack_protect_tls_canary_mem (bool);
> > +
> >
> > #if defined TREE_CODE
> > extern void arm_init_cumulative_args (CUMULATIVE_ARGS *, tree, rtx, tree);
> > diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
> > index c4ff06b087eb..6a659d81a6fe 100644
> > --- a/gcc/config/arm/arm.c
> > +++ b/gcc/config/arm/arm.c
> > @@ -829,6 +829,9 @@ static const struct attribute_spec arm_attribute_table[] =
> >
> > #undef TARGET_MD_ASM_ADJUST
> > #define TARGET_MD_ASM_ADJUST arm_md_asm_adjust
> > +
> > +#undef TARGET_STACK_PROTECT_GUARD
> > +#define TARGET_STACK_PROTECT_GUARD arm_stack_protect_guard
> >
> > /* Obstack for minipool constant handling.  */
> > static struct obstack minipool_obstack;
> > @@ -3155,6 +3158,26 @@ arm_option_override_internal (struct gcc_options *opts,
> >   if (TARGET_THUMB2_P (opts->x_target_flags))
> >     opts->x_inline_asm_unified = true;
> >
> > +  if (arm_stack_protector_guard == SSP_GLOBAL
> > +      && opts->x_arm_stack_protector_guard_offset_str)
> > +    {
> > +      error ("incompatible options %'-mstack-protector-guard=global%' and"
>
> Are the two  “%” in the above needed?
>

I get warnings about bare quotes otherwise, so yes AFAICT.

> > +          "%'-mstack-protector-guard-offset=%qs%’”,
> Are the two “%” in the above needed?
>
> > +          arm_stack_protector_guard_offset_str);
> > +    }
>
>
> > +
> > +  if (opts->x_arm_stack_protector_guard_offset_str)
> > +    {
> > +      char *end;
> > +      const char *str = arm_stack_protector_guard_offset_str;
> > +      errno = 0;
> > +      long offs = strtol (arm_stack_protector_guard_offset_str, &end, 0);
> > +      if (!*str || *end || errno)
> > +     error ("%qs is not a valid offset in %qs", str,
> > +            "-mstack-protector-guard-offset=");
> > +      arm_stack_protector_guard_offset = offs;
> > +    }
> > +
> > #ifdef SUBTARGET_OVERRIDE_INTERNAL_OPTIONS
> >   SUBTARGET_OVERRIDE_INTERNAL_OPTIONS;
> > #endif
> > @@ -3822,6 +3845,10 @@ arm_option_reconfigure_globals (void)
> >       else
> >       target_thread_pointer = TP_SOFT;
> >     }
> > +
> > +  if (arm_stack_protector_guard == SSP_TLSREG
> > +      && target_thread_pointer != TP_CP15)
> > +    error("%'-mstack-protector-guard=tls%' needs a hardware TLS register");
>
> Under what situation, this might happen?
>

If the user passes -mtp=soft -mstack-protector-guard=tls, we get in
the paradoxical situation where the stack protector uses the MRC
instruction directly, whereas the TLS variable accesses will call
__aeabi_read_tp to read the same value. This is sub-optimal at the
very least, but also unlikely to be what the user intended.

> > }
> >
> > /* Perform some validation between the desired architecture and the rest of the
> > @@ -8087,6 +8114,22 @@ legitimize_pic_address (rtx orig, machine_mode mode, rtx reg, rtx pic_reg,
> > }
> >
> >
> > +rtx
> > +arm_stack_protect_tls_canary_mem (bool reload)
> > +{
> > +  rtx tp = gen_reg_rtx (SImode);
> > +  if (reload)
> > +    emit_insn (gen_reload_tp_hard (tp));
> > +  else
> > +    emit_insn (gen_load_tp_hard (tp));
> > +
> > +  rtx reg = gen_reg_rtx (SImode);
> > +  rtx offset = GEN_INT (arm_stack_protector_guard_offset);
> > +  emit_set_insn (reg, gen_rtx_PLUS (SImode, tp, offset));
> > +  return gen_rtx_MEM (SImode, reg);
> > +}
>
> Could you add a comment for the above routine?
>

Yes.

> > +
> > +
> > /* Whether a register is callee saved or not.  This is necessary because high
> >    registers are marked as caller saved when optimizing for size on Thumb-1
> >    targets despite being callee saved in order to avoid using them.  */
> > @@ -34054,6 +34097,18 @@ arm_run_selftests (void)
> > #define TARGET_RUN_TARGET_SELFTESTS selftest::arm_run_selftests
> > #endif /* CHECKING_P */
> >
> > +/* Implement TARGET_STACK_PROTECT_GUARD. In case of a
> > +   global variable based guard use the default else
> > +   return a null tree.  */
> > +static tree
> > +arm_stack_protect_guard (void)
> > +{
> > +  if (arm_stack_protector_guard == SSP_GLOBAL)
> > +    return default_stack_protect_guard ();
> > +
> > +  return NULL_TREE;
> > +}
> > +
> > /* Worker function for TARGET_MD_ASM_ADJUST, while in thumb1 mode.
> >    Unlike the arm version, we do NOT implement asm flag outputs.  */
> >
> > diff --git a/gcc/config/arm/arm.md b/gcc/config/arm/arm.md
> > index 4adc976b8b67..d31349cd2614 100644
> > --- a/gcc/config/arm/arm.md
> > +++ b/gcc/config/arm/arm.md
> > @@ -9183,7 +9183,7 @@ (define_expand "stack_protect_combined_set"
> >                     UNSPEC_SP_SET))
> >       (clobber (match_scratch:SI 2 ""))
> >       (clobber (match_scratch:SI 3 ""))])]
> > -  ""
> > +  "arm_stack_protector_guard == SSP_GLOBAL"
> >   ""
> > )
> >
> > @@ -9267,7 +9267,7 @@ (define_expand "stack_protect_combined_test"
> >       (clobber (match_scratch:SI 3 ""))
> >       (clobber (match_scratch:SI 4 ""))
> >       (clobber (reg:CC CC_REGNUM))])]
> > -  ""
> > +  "arm_stack_protector_guard == SSP_GLOBAL"
> >   ""
> > )
> >
> > @@ -9361,6 +9361,64 @@ (define_insn "arm_stack_protect_test_insn"
> >    (set_attr "arch" "t,32")]
> > )
> >
> > +(define_expand "stack_protect_set"
> > +  [(match_operand:SI 0 "memory_operand")
> > +   (match_operand:SI 1 "memory_operand")]
> > +  "arm_stack_protector_guard == SSP_TLSREG"
> > +  "
> > +{
> > +  operands[1] = arm_stack_protect_tls_canary_mem (false /* reload */);
> > +  emit_insn (gen_stack_protect_set_tls (operands[0], operands[1]));
> > +  DONE;
> > +}"
> > +)
> > +
> > +;; DO NOT SPLIT THIS PATTERN.  It is important for security reasons that the
> > +;; canary value does not live beyond the life of this sequence.
> > +(define_insn "stack_protect_set_tls"
> > +  [(set (match_operand:SI 0 "memory_operand" "=m")
> > +       (unspec:SI [(match_operand:SI 1 "memory_operand" "m")]
> > +        UNSPEC_SP_SET))
> > +   (set (match_scratch:SI 2 "=&r") (const_int 0))]
> > +  ""
> > +  "ldr\\t%2, %1\;str\\t%2, %0\;mov\t%2, #0"
> > +  [(set_attr "length" "12")
> > +   (set_attr "conds" "nocond")
> > +   (set_attr "type" "multiple")]
> > +)
> > +
> > +(define_expand "stack_protect_test"
> > +  [(match_operand:SI 0 "memory_operand")
> > +   (match_operand:SI 1 "memory_operand")
> > +   (match_operand:SI 2)]
> > +  "arm_stack_protector_guard == SSP_TLSREG"
> > +  "
> > +{
> > +  operands[1] = arm_stack_protect_tls_canary_mem (true /* reload */);
> > +  emit_insn (gen_stack_protect_test_tls (operands[0], operands[1]));
> > +
> > +  rtx cc_reg = gen_rtx_REG (CC_Zmode, CC_REGNUM);
> > +  rtx eq = gen_rtx_EQ (CC_Zmode, cc_reg, const0_rtx);
> > +  emit_jump_insn (gen_arm_cond_branch (operands[2], eq, cc_reg));
> > +  DONE;
> > +}"
> > +)
> > +
> > +(define_insn "stack_protect_test_tls"
> > +  [(set (reg:CC_Z CC_REGNUM)
> > +     (compare:CC_Z (unspec:SI [(match_operand:SI 0 "memory_operand" "m")
> > +                               (match_operand:SI 1 "memory_operand" "m")]
> > +                              UNSPEC_SP_TEST)
> > +                   (const_int 0)))
> > +   (clobber (match_scratch:SI 2 "=&r"))
> > +   (clobber (match_scratch:SI 3 "=&r"))]
> > +  ""
> > +  "ldr\t%2, %0\;ldr\t%3, %1\;eors\t%2, %3, %2\;mov\t%3, #0"
> > +  [(set_attr "length" "16")
> > +   (set_attr "conds" "set")
> > +   (set_attr "type" "multiple")]
> > +)
> > +
> > (define_expand "casesi"
> >   [(match_operand:SI 0 "s_register_operand")  ; index to jump on
> >    (match_operand:SI 1 "const_int_operand")   ; lower bound
> > @@ -12133,6 +12191,15 @@ (define_insn "load_tp_hard"
> >    (set_attr "type" "mrs")]
> > )
> >
> > +;; Used by the TLS register based stack protector
> > +(define_insn "reload_tp_hard"
> > +  [(set (match_operand:SI 0 "register_operand" "=r")
> > +     (unspec_volatile [(const_int 0)] VUNSPEC_MRC))]
> > +  "TARGET_HARD_TP"
> > +  "mrc\\tp15, 0, %0, c13, c0, 3\\t@ reload_tp_hard"
> > +  [(set_attr "type" "mrs")]
> > +)
> > +
> > ;; Doesn't clobber R1-R3.  Must use r0 for the first operand.
> > (define_insn "load_tp_soft_fdpic"
> >   [(set (reg:SI 0) (unspec:SI [(const_int 0)] UNSPEC_TLS))
> > diff --git a/gcc/config/arm/arm.opt b/gcc/config/arm/arm.opt
> > index a7677eeb45c8..4b3e17bc319c 100644
> > --- a/gcc/config/arm/arm.opt
> > +++ b/gcc/config/arm/arm.opt
> > @@ -311,3 +311,25 @@ Generate code which uses the core registers only (r0-r14).
> > mfdpic
> > Target Mask(FDPIC)
> > Enable Function Descriptor PIC mode.
> > +
> > +mstack-protector-guard=
> > +Target RejectNegative Joined Enum(stack_protector_guard) Var(arm_stack_protector_guard) Init(SSP_GLOBAL)
> > +Use given stack-protector guard.
> > +
> > +Enum
> > +Name(stack_protector_guard) Type(enum stack_protector_guard)
> > +Valid arguments to -mstack-protector-guard=:
> > +
> > +EnumValue
> > +Enum(stack_protector_guard) String(tls) Value(SSP_TLSREG)
> > +
> > +EnumValue
> > +Enum(stack_protector_guard) String(global) Value(SSP_GLOBAL)
> > +
> > +mstack-protector-guard-offset=
> > +Target Joined RejectNegative String Var(arm_stack_protector_guard_offset_str)
> > +Use an immediate to offset from the TLS register. This option is for use with
> > +fstack-protector-guard=tls and not for use in user-land code.
> > +
> > +TargetVariable
> > +long arm_stack_protector_guard_offset = 0
> > diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> > index 71992b8c5974..46d009376018 100644
> > --- a/gcc/doc/invoke.texi
> > +++ b/gcc/doc/invoke.texi
> > @@ -810,6 +810,7 @@ Objective-C and Objective-C++ Dialects}.
> > -mpure-code @gol
> > -mcmse @gol
> > -mfix-cmse-cve-2021-35465 @gol
> > +-mstack-protector-guard=@...{guard} -mstack-protector-guard-offset=@...{offset} @gol
> > -mfdpic}
> >
> > @emph{AVR Options}
> > @@ -20946,6 +20947,14 @@ enabled by default when the option @option{-mcpu=} is used with
> > @code{cortex-m33}, @code{cortex-m35p} or @code{cortex-m55}.  The option
> > @option{-mno-fix-cmse-cve-2021-35465} can be used to disable the mitigation.
> >
> > +@...m -mstack-protector-guard=@...{guard}
> > +@...mx -mstack-protector-guard-offset=@...{offset}
> > +@...ndex mstack-protector-guard
> > +@...ndex mstack-protector-guard-offset
> > +Generate stack protection code using canary at @var{guard}.  Supported
> > +locations are @samp{global} for a global canary or @samp{tls} for a
> > +canary accessible via the TLS register.
> > +
>
>
> There is no documentation for -mstack-protector-guard-offset here, and you might want to explain the relationship between -mstack-protector-guard= and it.
>

OK, got it.

Thanks a lot for the review!

-- 
Ard.

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