Message-ID: <CAMj1kXEgHCT4Z_T70QXtoYg1jMrrmw7AmpZu3DDmmXqe9CiB+Q@mail.gmail.com>
Date:   Wed, 17 Nov 2021 18:12:51 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     linux-hardening@...r.kernel.org,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>
Cc:     Keith Packard <keithpac@...zon.com>, thomas.preudhomme@...est.fr,
        adhemerval.zanella@...aro.org, Qing Zhao <qing.zhao@...cle.com>,
        Richard Sandiford <richard.sandiford@....com>,
        Kyrylo Tkachov <kyryo.tkachov@....com>, gcc-patches@....gnu.org
Subject: Re: [PATCH v5 1/1] [ARM] Add support for TLS register based stack
 protector canary access

(+ Ramana)

On Mon, 15 Nov 2021 at 19:04, 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.
>
> 2021-11-15 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
>
> gcc/testsuite/ChangeLog:
>
>         * gcc.target/arm/stack-protector-7.c: New test.
>         * gcc.target/arm/stack-protector-8.c: New test.
>
> 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                              | 11 +++
>  gcc/testsuite/gcc.target/arm/stack-protector-7.c | 10 +++
>  gcc/testsuite/gcc.target/arm/stack-protector-8.c |  5 ++
>  8 files changed, 180 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 a5b403eb3e49..e5077348ce07 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;
> @@ -3176,6 +3179,26 @@ 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"
> +            "%'-mstack-protector-guard-offset=%qs%'",
> +            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
> @@ -3843,6 +3866,9 @@ arm_option_reconfigure_globals (void)
>        else
>         target_thread_pointer = TP_SOFT;
>      }
> +
> +  if (!TARGET_HARD_TP && arm_stack_protector_guard == SSP_TLSREG)
> +    error("%'-mstack-protector-guard=tls%' needs a hardware TLS register");
>  }
>
>  /* Perform some validation between the desired architecture and the rest of the
> @@ -8108,6 +8134,23 @@ legitimize_pic_address (rtx orig, machine_mode mode, rtx reg, rtx pic_reg,
>  }
>
>
> +/* Generate insns that produce the address of the stack canary */
> +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);
> +}
> +
> +
>  /* 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.  */
> @@ -34075,6 +34118,18 @@ 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..45a54d63f5b3 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" "unconditional")
> +   (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:SI [(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 6070288856c0..13bebbcbf0fb 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -816,6 +816,7 @@ 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}
> @@ -21099,6 +21100,16 @@ 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. The option
> +@...ion{-mstack-protector-guard-offset=} is for use with
> +@...ion{-fstack-protector-guard=tls} and not for use in user-land code.
> +
>  @item -mfdpic
>  @itemx -mno-fdpic
>  @opindex mfdpic
> diff --git a/gcc/testsuite/gcc.target/arm/stack-protector-7.c b/gcc/testsuite/gcc.target/arm/stack-protector-7.c
> new file mode 100644
> index 000000000000..874648060d8d
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/arm/stack-protector-7.c
> @@ -0,0 +1,10 @@
> +/* { dg-do compile } */
> +/* { dg-options "-march=armv7-a -mfpu=vfp -fstack-protector-all -Os -mstack-protector-guard=tls -mstack-protector-guard-offset=1296 -mtp=cp15" } */
> +
> +#include "stack-protector-5.c"
> +
> +/* See the comment in stack-protector-5.c.  */
> +/* { dg-final { scan-assembler-times {\tstr\t} 1 } } */
> +/* Expect two TLS register accesses and two occurrences of the offset */
> +/* { dg-final { scan-assembler-times {\tmrc\t} 2 } } */
> +/* { dg-final { scan-assembler-times {1296} 2 } } */
> diff --git a/gcc/testsuite/gcc.target/arm/stack-protector-8.c b/gcc/testsuite/gcc.target/arm/stack-protector-8.c
> new file mode 100644
> index 000000000000..bcb2f6b82bdf
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/arm/stack-protector-8.c
> @@ -0,0 +1,5 @@
> +/* { dg-do compile } */
> +/* { dg-error "needs a hardware TLS register" "missing error when using TLS stack protector without hardware TLS register" { target *-*-* } 0 } */
> +/* { dg-options "-fstack-protector-all -Os -mstack-protector-guard=tls -mtp=soft" } */
> +
> +int foo;
> --
> 2.30.2
>

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