0x2c binary options
The aim of this post is to illustrate the analysis of Linux kernel crashes by studying a few real-life examples. The examples are coming from a MIPS platform, but the general approach is applicable to other architectures. If a binary was built with debugging information, objdump -S can display source code intermixed with disassembly 1. Also, addr2line can be used to match addresses with source code file names and lines. In order to interpret disassembly, we need to have the MIPS Instruction Reference and the Compiler Register Usage information at hand 2so please keep these pages open while reading further material.
If you are not familiar with how the virtual memory space is divided on MIPS, please refer to 'Virtual Memory Layout' 3 in the last section. On CPU 0 a load or store instruction at address epc accessed a virtual address 0x There was no valid virtual to physical address translation available - hence, the crash. In the middle of the report the same virtual address is shown as:.
This mechanism does what assert does for user-space applications. To simplify reading, each of them has a mnemonic name in assembler code. Now it's time to take a quick look at Compiler Register Usage. An ideal case is to have a complete dump of the memory used by the kernel. That would allow us to restore the environment - to see the content of local variables, various kernel data structures, etc.
Kdump can do this no MIPS support at the moment. Nevertheless, in many cases the 0x2c binary options of the registers alone reveals enough information to understand a problem. The content of Status and Cause registers may be very useful in some cases, but we won't consider them here. In practice, ra usually points either to a caller of the function where epc belongs to or to the same function as epc.
An invalid address in ra can indicate stack corruption at least for non-leaf functions. In any case, these names can be located with objdump. In most cases, ra points to the 2nd instruction that follows an instruction representing a function call usually jal or jalr.
A function call is at 0xf9c The instruction at 0xf9c1c is located in the delay slot of jalr and is executed before any instruction in the called function.
Delay slots of jalr are 0x2c binary options used to initialize one of the function arguments. If the called function above has at least 4 arguments, its 4th argument will be 1. Branch instructions, like bltz branch on less than zeroare another example of instructions with delay slots.
We mentioned earlier that both epc and ra may point to the same function. To illustrate this case, let's suppose that a crash occurs at 0xf9c24 in the above disassembly. Provided that a function call at 0xf9c18 took place, ra would point to 0xf9c20 inside the same function as epc. In an ideal world where kernels and, especially, kernel modules behave well, user-space actions can never trigger a kernel crash.
No matter what these actions are. Kernel-mode tasks have full privileges to cause havoc though. This is a partial dump of the kernel-space stack of the current task, which we have discussed in the previous section. It does not always represent a genuine call trace though. It contains all the values from stack that 0x2c binary options like valid return addresses. So there can be 'ghost' traces of previously run 0x2c binary options completely unrelated functions.
If it was not the case, we may note that both epc and ra belong to kseg0 3so we may expect to find them inside the kernel image vmlinux. It's also clear that a function being 0x2c binary options is strlen. In cases when the address of a called function is not known at build time kernel modulesdisassembly 0x2c binary options look as follows:. The first 2 instructions are changed by the loader at run time. In such cases, don't get confused when disassembly and the Code: Usually, there is a remote resemblance though.
For instance, the instructions above might have been changed as follows:. 0x2c binary options checks can be also applied as sanity checks to ensure 0x2c binary options have got the right image for disassembly. Now, a0 is supposed to hold the 1st and only argument of strlen Compiler Register Usage. Given that epc points to 0x2c binary options 1st instruction, a0 has not yet been reused for anything else inside strlen. This can be verified by analyzing an instructions flow.
But before doing so, we should consider a few more aspects common to all functions. At the 0x2c binary options of most of the functions, there is a sequence of instruction called "prologue".
The first instruction creates a stack frame by reserving 0x2c binary options on the stack. Same applies to 0x2c binary options for non-leaf functions those calling other functions. Obviously, local variables also reside on this stack frame.
The content of reused registers is restored. The stack frame is deleted - usually, by the last instruction in a delay slot of jr. Finally, control is given back to the caller by jr ra. Stack corruptions may overwrite a value corresponding to raresulting in the control being given 0x2c binary options unexpected places unless this is a result of a deliberate security attack.
This is likely to result in " Unable to handle kernel paging request ", 0x2c binary options Unaligned access "or " Invalid instruction ". Quite often in such cases epc is equal to ra or close to it or to both ra and BadVA. Moreover, a function may have more than one "epilogue". The instructions initializing a0-a3 do not have to be placed immediately next to the jal instruction, but usually they are in some proximity.
A few instructions above the actual call see remarks - at 0xbwe can see that a0 is being loaded with the content of a1. After examining the remaining instructions it becomes clear that a1 still holds 0x2c binary options initial value that corresponds to the 2nd argument of strlcpy dst, src, len.
Now we can update our working hypothesis. It looks like strlcpy has been called with its 2nd argument, srcbeing NULL. This is in accordance with our hypothesis indeed. Recall the remarks above regarding the validity of call traces. Basic sanity checks won't take much time. Having disassembly intermixed with source code is helpful here. In any case, there is obviously a 0x2c binary options as to how 0x2c binary options "in the past" we would be able to look by analyzing a crash report even if we had a complete memory dump.
Nevertheless, the results of this analysis - if not sufficient to reveal a root cause - are usually very helpful in further debugging. Next step is to 0x2c binary options the flow of instructions to trace the source of the value in a0.
A full listing is not provided here, but 0x2c binary options it revealed is that a0 is used read-only. At epc it still holds the 1st input argument of the function. Moreover, there are no explicit validity checks prior to its use. Thus, the 1st argument is expected to be a valid address. There are 2 function calls here. The 0x2c binary options one, at line 3seems to have a single argument which gets initialized at line 4. The second one, at line 10takes 3 arguments that are initialized at lines 75and 11 respectively.
A CPU has tried to fetch an instruction at 0xcbut this is 0x2c binary options a valid kernel-space address. Well, some knowledge of the kernel internals would be helpful here. So where do we go from here? Memory corruptions often 0x2c binary options in seemingly unrelated crashes: What common 0x2c binary options is that all these crashes may look "weird".
Having multiple crashes in different parts of the core kernel code is usually a good indicator too. Of course, it's always possible to overlook something.
So the larger a set of crashes from which a conclusion is drawn, the better. In this particular case, 0x2c binary options was another crash in the same location among a dozen of crashes in yet other areas where the syscall number and epc were 0x68 and 0xd8 correspondingly. Maybe it's just a coincidence but is worth taking into account. Now, 0x2c binary options about the content of epc?
Do we actually know how the correct values would look like? We can look them up with nm or from disassembly:.
Is there another pattern? Yes, the only difference between good and bad values is in the high bit 0x This missing-high-bit theory had explanatory power when applied to some of the other "weird" crashes for which it was possible to infer the good value. How could this bit be cleared? In the end, it has been found that DDR timing settings were not properly set in the bootloader.
Alternatively, you can reproduce the original binary if possible with debugging information enabled and then use'-dS'. Be careful though to double-check that the addresses you are interested in correspond 0x2c binary options the same instructions in both original and new disassembly files. Be sure to verify the options used by your toolchain, if in doubt.
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