Hi Everyone,
I'm seeing some weird memory corruption that I have been able to isolate
to arch_ptrace() [arch/powerpc/kernel/ptrace.c] and PTRACE_POKEUSR. I am
on PowerPC 32 (MPC8378), kernel 4.9.179.
It's not very easy for me to test on the latest kernel, but I guess
little has changed since 4.9 in either the architecture specific ptrace
code or PowerPC register data structures.
What happens is that gdb calls ptrace(PTRACE_POKEUSER) with addr=0x158.
This goes down to arch_ptrace() [arch/powerpc/kernel/ptrace.c], inside
`case PTRACE_POKEUSR`, on the branch that does this:
memcpy(&child->thread.TS_FPR(fpidx), &data,
sizeof(long));
where:
index = addr >> 2 = 0x56 = 86
fpidx = index - PT_FPR0 = 86 - 48 = 38
&child->thread.TS_FPR(fpidx) = (void *)child + 1296
offsetof(struct task_struct, thread) = 960
sizeof(struct thread_struct) = 336
sizeof(struct task_struct) = 1296
In other words, the memcpy() call writes just beyond thread_struct
(which is also beyond task_struct, for that matter).
This should never get past the bounds checks for `index`, so perhaps
there is a mismatch between ptrace macros and the actual register data
structures layout.
I will continue to investigate, but I'm not familiar with the PowerPC
registers so it will take a while before I make sense of all the data
structures and macros. Hopefully this rings a bell to someone who is
already familiar with those and could figure out quickly what the
problem is.
Best regards,
Radu Rendec
Le 05/06/2019 à 23:45, Radu Rendec a écrit :
Hi Everyone,
I'm seeing some weird memory corruption that I have been able to isolate
to arch_ptrace() [arch/powerpc/kernel/ptrace.c] and PTRACE_POKEUSR. I am
on PowerPC 32 (MPC8378), kernel 4.9.179.
It's not very easy for me to test on the latest kernel, but I guess
little has changed since 4.9 in either the architecture specific ptrace
code or PowerPC register data structures.
What happens is that gdb calls ptrace(PTRACE_POKEUSER) with addr=0x158.
This goes down to arch_ptrace() [arch/powerpc/kernel/ptrace.c], inside
`case PTRACE_POKEUSR`, on the branch that does this:
memcpy(&child->thread.TS_FPR(fpidx), &data,
sizeof(long));
where:
index = addr >> 2 = 0x56 = 86
fpidx = index - PT_FPR0 = 86 - 48 = 38
In struct thread_fp_state, fpr field is u64, so I guess we should have
the following on PPC32:
fpidx = (index - PT_FPR0) >> 1;
Christophe
&child->thread.TS_FPR(fpidx) = (void *)child + 1296
offsetof(struct task_struct, thread) = 960
sizeof(struct thread_struct) = 336
sizeof(struct task_struct) = 1296
In other words, the memcpy() call writes just beyond thread_struct
(which is also beyond task_struct, for that matter).
This should never get past the bounds checks for `index`, so perhaps
there is a mismatch between ptrace macros and the actual register data
structures layout.
I will continue to investigate, but I'm not familiar with the PowerPC
registers so it will take a while before I make sense of all the data
structures and macros. Hopefully this rings a bell to someone who is
already familiar with those and could figure out quickly what the
problem is.
Best regards,
Radu Rendec
On Thu, 2019-06-06 at 07:15 +0200, Christophe Leroy wrote:
Le 05/06/2019 à 23:45, Radu Rendec a écrit :
quoted
Hi Everyone,
I'm seeing some weird memory corruption that I have been able to isolate
to arch_ptrace() [arch/powerpc/kernel/ptrace.c] and PTRACE_POKEUSR. I am
on PowerPC 32 (MPC8378), kernel 4.9.179.
It's not very easy for me to test on the latest kernel, but I guess
little has changed since 4.9 in either the architecture specific ptrace
code or PowerPC register data structures.
What happens is that gdb calls ptrace(PTRACE_POKEUSER) with addr=0x158.
This goes down to arch_ptrace() [arch/powerpc/kernel/ptrace.c], inside
`case PTRACE_POKEUSR`, on the branch that does this:
memcpy(&child->thread.TS_FPR(fpidx), &data,
sizeof(long));
where:
index = addr >> 2 = 0x56 = 86
fpidx = index - PT_FPR0 = 86 - 48 = 38
In struct thread_fp_state, fpr field is u64, so I guess we should have
the following on PPC32:
fpidx = (index - PT_FPR0) >> 1;
I guess this would only apply to PPC32, since everything up to fpidx is
calculated in units of sizeof(long) - which is 4 on PPC32 and 8 on
PPC64. But fpr[0:31] is always u64.
It also looks odd that only sizeof(long) bytes are ever copied for any
given fpr[fpidx], which means one half of the u64 is never accessible on
PPC32.
Ont other thing I don't get is the "+1" in the definition of PT_FPSCR
for PPC32:
#define PT_FPSCR (PT_FPR0 + 2*32 + 1)
Looking at struct thread_fp_state, fpscr follows immediately after
fpr[31]. Is the FPSCR register only 32-bit on PPC32? Is it stored in the
2nd half of (struct thread_fp_state).fpscr? This line:
child->thread.fp_state.fpscr = data;
suggests so. And in that case, the "+1" in PT_FPSCR makes sense, but
only for big endian: assigning `data` (which is "long", 32-bit) to the
`fpscr` field (which is "u64") would go to the higher address, which is
indeed "+1" in units of 32-bit words.
Then there is also a problem is the condition that determines whether
memcpy() is used to access one of the fpr[0:31] or fpscr is assigned
directly:
if (fpidx < (PT_FPSCR - PT_FPR0))
The case when the supplied addr points to the lower half of fpscr (which
is unused on PPC32?) erroneously indexes into fpr[0:31].
Is there any documentation of what "addr" is supposed to mean?
quoted
&child->thread.TS_FPR(fpidx) = (void *)child + 1296
offsetof(struct task_struct, thread) = 960
sizeof(struct thread_struct) = 336
sizeof(struct task_struct) = 1296
In other words, the memcpy() call writes just beyond thread_struct
(which is also beyond task_struct, for that matter).
This should never get past the bounds checks for `index`, so perhaps
there is a mismatch between ptrace macros and the actual register data
structures layout.
I will continue to investigate, but I'm not familiar with the PowerPC
registers so it will take a while before I make sense of all the data
structures and macros. Hopefully this rings a bell to someone who is
already familiar with those and could figure out quickly what the
problem is.