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[aarch64] Enable pointer authentication support for aarch64 bare meta…
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…l/kernel mode addresses

At the moment GDB only handles pointer authentication (pauth) for userspace
addresses and if we're debugging a Linux-hosted program.

The Linux Kernel can be configured to use pauth instructions for some
additional security hardening, but GDB doesn't handle this well.

To overcome this limitation, GDB needs a couple things:

1 - The target needs to advertise pauth support.
2 - The hook to remove non-address bits from a pointer needs to be registered
    in aarch64-tdep.c as opposed to aarch64-linux-tdep.c.

There is a patch for QEMU that addresses the first point, and it makes
QEMU's gdbstub expose a couple more pauth mask registers, so overall we will
have up to 4 pauth masks (2 masks or 4 masks):

pauth_dmask
pauth_cmask
pauth_dmask_high
pauth_cmask_high

pauth_dmask and pauth_cmask are the masks used to remove pauth signatures
from userspace addresses. pauth_dmask_high and pauth_cmask_high masks are used
to remove pauth signatures from kernel addresses.

The second point is easily addressed by moving code around.

When debugging a Linux Kernel built with pauth with an unpatched GDB, we get
the following backtrace:

 #0  __fput (file=0xffff0000c17a6400) at /repos/linux/fs/file_table.c:296
 #1  0xffff8000082bd1f0 in ____fput (work=<optimized out>) at /repos/linux/fs/file_table.c:348
 #2  0x30008000080ade30 [PAC] in ?? ()
 #3  0x30d48000080ade30 in ?? ()
 Backtrace stopped: previous frame identical to this frame (corrupt stack?)

With a patched GDB, we get something a lot more meaningful:

 #0  __fput (file=0xffff0000c1bcfa00) at /repos/linux/fs/file_table.c:296
 #1  0xffff8000082bd1f0 in ____fput (work=<optimized out>) at /repos/linux/fs/file_table.c:348
 #2  0xffff8000080ade30 [PAC] in task_work_run () at /repos/linux/kernel/task_work.c:179
 #3  0xffff80000801db90 [PAC] in resume_user_mode_work (regs=0xffff80000a96beb0) at /repos/linux/include/linux/resume_user_mode.h:49
 #4  do_notify_resume (regs=regs@entry=0xffff80000a96beb0, thread_flags=4) at /repos/linux/arch/arm64/kernel/signal.c:1127
 #5  0xffff800008fb9974 [PAC] in prepare_exit_to_user_mode (regs=0xffff80000a96beb0) at /repos/linux/arch/arm64/kernel/entry-common.c:137
 #6  exit_to_user_mode (regs=0xffff80000a96beb0) at /repos/linux/arch/arm64/kernel/entry-common.c:142
 #7  el0_svc (regs=0xffff80000a96beb0) at /repos/linux/arch/arm64/kernel/entry-common.c:638
 #8  0xffff800008fb9d34 [PAC] in el0t_64_sync_handler (regs=<optimized out>) at /repos/linux/arch/arm64/kernel/entry-common.c:655
 #9  0xffff800008011548 [PAC] in el0t_64_sync () at /repos/linux/arch/arm64/kernel/entry.S:586
 Backtrace stopped: Cannot access memory at address 0xffff80000a96c0c8
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@luis-machado-arm
luis-machado-arm committed Feb 21, 2023
1 parent fd7f742 commit 6d00208
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Showing 4 changed files with 100 additions and 51 deletions.
40 changes: 0 additions & 40 deletions gdb/aarch64-linux-tdep.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -1980,40 +1980,6 @@ aarch64_linux_decode_memtag_section (struct gdbarch *gdbarch,
return tags;
}

/* AArch64 implementation of the remove_non_address_bits gdbarch hook. Remove
non address bits from a pointer value. */

static CORE_ADDR
aarch64_remove_non_address_bits (struct gdbarch *gdbarch, CORE_ADDR pointer)
{
aarch64_gdbarch_tdep *tdep = gdbarch_tdep<aarch64_gdbarch_tdep> (gdbarch);

/* By default, we assume TBI and discard the top 8 bits plus the VA range
select bit (55). */
CORE_ADDR mask = AARCH64_TOP_BITS_MASK;

if (tdep->has_pauth ())
{
/* Fetch the PAC masks. These masks are per-process, so we can just
fetch data from whatever thread we have at the moment.
Also, we have both a code mask and a data mask. For now they are the
same, but this may change in the future. */
struct regcache *regs = get_current_regcache ();
CORE_ADDR cmask, dmask;

if (regs->cooked_read (tdep->pauth_reg_base, &dmask) != REG_VALID)
dmask = mask;

if (regs->cooked_read (tdep->pauth_reg_base + 1, &cmask) != REG_VALID)
cmask = mask;

mask |= aarch64_mask_from_pac_registers (cmask, dmask);
}

return aarch64_remove_top_bits (pointer, mask);
}

static void
aarch64_linux_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch)
{
Expand Down Expand Up @@ -2066,12 +2032,6 @@ aarch64_linux_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch)
/* Syscall record. */
tdep->aarch64_syscall_record = aarch64_linux_syscall_record;

/* The top byte of a user space address known as the "tag",
is ignored by the kernel and can be regarded as additional
data associated with the address. */
set_gdbarch_remove_non_address_bits (gdbarch,
aarch64_remove_non_address_bits);

/* MTE-specific settings and hooks. */
if (tdep->has_mte ())
{
Expand Down
103 changes: 92 additions & 11 deletions gdb/aarch64-tdep.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -137,10 +137,14 @@ static const char *const aarch64_sve_register_names[] =

static const char *const aarch64_pauth_register_names[] =
{
/* Authentication mask for data pointer. */
/* Authentication mask for data pointer, low half/user pointers. */
"pauth_dmask",
/* Authentication mask for code pointer. */
"pauth_cmask"
/* Authentication mask for code pointer, low half/user pointers. */
"pauth_cmask",
/* Authentication mask for data pointer, high half / kernel pointers. */
"pauth_dmask_high",
/* Authentication mask for code pointer, high half / kernel pointers. */
"pauth_cmask_high"
};

static const char *const aarch64_mte_register_names[] =
Expand Down Expand Up @@ -228,9 +232,19 @@ aarch64_frame_unmask_lr (aarch64_gdbarch_tdep *tdep,
&& frame_unwind_register_unsigned (this_frame,
tdep->ra_sign_state_regnum))
{
int cmask_num = AARCH64_PAUTH_CMASK_REGNUM (tdep->pauth_reg_base);
CORE_ADDR cmask = frame_unwind_register_unsigned (this_frame, cmask_num);
addr = addr & ~cmask;
/* VA range select (bit 55) tells us whether to use the low half masks
or the high half masks. */
int cmask_num;
if (tdep->pauth_reg_count > 2 && addr & VA_RANGE_SELECT_BIT_MASK)
cmask_num = AARCH64_PAUTH_CMASK_HIGH_REGNUM (tdep->pauth_reg_base);
else
cmask_num = AARCH64_PAUTH_CMASK_REGNUM (tdep->pauth_reg_base);

/* By default, we assume TBI and discard the top 8 bits plus the VA range
select bit (55). */
CORE_ADDR mask = AARCH64_TOP_BITS_MASK;
mask |= frame_unwind_register_unsigned (this_frame, cmask_num);
addr = aarch64_remove_top_bits (addr, mask);

/* Record in the frame that the link register required unmasking. */
set_frame_previous_pc_masked (this_frame);
Expand Down Expand Up @@ -1326,8 +1340,8 @@ aarch64_dwarf2_frame_init_reg (struct gdbarch *gdbarch, int regnum,
reg->loc.exp.len = 1;
return;
}
else if (regnum == AARCH64_PAUTH_DMASK_REGNUM (tdep->pauth_reg_base)
|| regnum == AARCH64_PAUTH_CMASK_REGNUM (tdep->pauth_reg_base))
else if (regnum >= tdep->pauth_reg_base
&& regnum < tdep->pauth_reg_base + tdep->pauth_reg_count)
{
reg->how = DWARF2_FRAME_REG_SAME_VALUE;
return;
Expand Down Expand Up @@ -3507,8 +3521,8 @@ aarch64_cannot_store_register (struct gdbarch *gdbarch, int regnum)
return 0;

/* Pointer authentication registers are read-only. */
return (regnum == AARCH64_PAUTH_DMASK_REGNUM (tdep->pauth_reg_base)
|| regnum == AARCH64_PAUTH_CMASK_REGNUM (tdep->pauth_reg_base));
return (regnum >= tdep->pauth_reg_base
&& regnum < tdep->pauth_reg_base + tdep->pauth_reg_count);
}

/* Implement the stack_frame_destroyed_p gdbarch method. */
Expand Down Expand Up @@ -3536,6 +3550,51 @@ aarch64_stack_frame_destroyed_p (struct gdbarch *gdbarch, CORE_ADDR pc)
return streq (inst.opcode->name, "ret");
}

/* AArch64 implementation of the remove_non_address_bits gdbarch hook. Remove
non address bits from a pointer value. */

static CORE_ADDR
aarch64_remove_non_address_bits (struct gdbarch *gdbarch, CORE_ADDR pointer)
{
aarch64_gdbarch_tdep *tdep = gdbarch_tdep<aarch64_gdbarch_tdep> (gdbarch);

/* By default, we assume TBI and discard the top 8 bits plus the VA range
select bit (55). */
CORE_ADDR mask = AARCH64_TOP_BITS_MASK;

if (tdep->has_pauth ())
{
/* Fetch the PAC masks. These masks are per-process, so we can just
fetch data from whatever thread we have at the moment.
Also, we have both a code mask and a data mask. For now they are the
same, but this may change in the future. */
struct regcache *regs = get_current_regcache ();
CORE_ADDR cmask, dmask;
int dmask_regnum = AARCH64_PAUTH_DMASK_REGNUM (tdep->pauth_reg_base);
int cmask_regnum = AARCH64_PAUTH_CMASK_REGNUM (tdep->pauth_reg_base);

/* If we have a kernel address and we have kernel-mode address mask
registers, use those instead. */
if (tdep->pauth_reg_count > 2
&& pointer & VA_RANGE_SELECT_BIT_MASK)
{
dmask_regnum = AARCH64_PAUTH_DMASK_HIGH_REGNUM (tdep->pauth_reg_base);
cmask_regnum = AARCH64_PAUTH_CMASK_HIGH_REGNUM (tdep->pauth_reg_base);
}

if (regs->cooked_read (dmask_regnum, &dmask) != REG_VALID)
dmask = mask;

if (regs->cooked_read (cmask_regnum, &cmask) != REG_VALID)
cmask = mask;

mask |= aarch64_mask_from_pac_registers (cmask, dmask);
}

return aarch64_remove_top_bits (pointer, mask);
}

/* Initialize the current architecture based on INFO. If possible,
re-use an architecture from ARCHES, which is a list of
architectures already created during this debugging session.
Expand Down Expand Up @@ -3674,19 +3733,35 @@ aarch64_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
}

/* Add the pauth registers. */
int pauth_masks = 0;
if (feature_pauth != NULL)
{
first_pauth_regnum = num_regs;
ra_sign_state_offset = num_pseudo_regs;

/* Size of the expected register set with all 4 masks. */
int set_size = ARRAY_SIZE (aarch64_pauth_register_names);

/* QEMU exposes a couple additional masks for the high half of the
address. We should either have 2 registers or 4 registers. */
if (tdesc_unnumbered_register (feature_pauth,
"pauth_dmask_high") == 0)
{
/* We did not find pauth_dmask_high, assume we only have
2 masks. We are not dealing with QEMU/Emulators then. */
set_size -= 2;
}

/* Validate the descriptor provides the mandatory PAUTH registers and
allocate their numbers. */
for (i = 0; i < ARRAY_SIZE (aarch64_pauth_register_names); i++)
for (i = 0; i < set_size; i++)
valid_p &= tdesc_numbered_register (feature_pauth, tdesc_data.get (),
first_pauth_regnum + i,
aarch64_pauth_register_names[i]);

num_regs += i;
num_pseudo_regs += 1; /* Count RA_STATE pseudo register. */
pauth_masks = set_size;
}

/* Add the MTE registers. */
Expand Down Expand Up @@ -3722,6 +3797,7 @@ aarch64_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
tdep->jb_elt_size = 8;
tdep->vq = vq;
tdep->pauth_reg_base = first_pauth_regnum;
tdep->pauth_reg_count = pauth_masks;
tdep->ra_sign_state_regnum = -1;
tdep->mte_reg_base = first_mte_regnum;
tdep->tls_regnum_base = first_tls_regnum;
Expand Down Expand Up @@ -3838,6 +3914,11 @@ aarch64_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
if (tdep->has_pauth ())
tdep->ra_sign_state_regnum = ra_sign_state_offset + num_regs;

/* Architecture hook to remove bits of a pointer that are not part of the
address, like memory tags (MTE) and pointer authentication signatures. */
set_gdbarch_remove_non_address_bits (gdbarch,
aarch64_remove_non_address_bits);

/* Add standard register aliases. */
for (i = 0; i < ARRAY_SIZE (aarch64_register_aliases); i++)
user_reg_add (gdbarch, aarch64_register_aliases[i].name,
Expand Down
2 changes: 2 additions & 0 deletions gdb/aarch64-tdep.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -94,6 +94,8 @@ struct aarch64_gdbarch_tdep : gdbarch_tdep_base
}

int pauth_reg_base = 0;
/* Number of pauth masks. */
int pauth_reg_count = 0;
int ra_sign_state_regnum = 0;

/* Returns true if the target supports pauth. */
Expand Down
6 changes: 6 additions & 0 deletions gdb/arch/aarch64.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -132,6 +132,12 @@ enum aarch64_regnum

#define AARCH64_PAUTH_DMASK_REGNUM(pauth_reg_base) (pauth_reg_base)
#define AARCH64_PAUTH_CMASK_REGNUM(pauth_reg_base) (pauth_reg_base + 1)
/* The high versions of these masks are used for bare metal/kernel-mode pointer
authentication support. */
#define AARCH64_PAUTH_DMASK_HIGH_REGNUM(pauth_reg_base) (pauth_reg_base + 2)
#define AARCH64_PAUTH_CMASK_HIGH_REGNUM(pauth_reg_base) (pauth_reg_base + 3)

/* This size is only meant for Linux, not bare metal. QEMU exposes 4 masks. */
#define AARCH64_PAUTH_REGS_SIZE (16)

#define AARCH64_X_REGS_NUM 31
Expand Down

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