Re: [PATCH] selftests/x86/lam: Zero out buffer for readlink()

[Binbin Wu <binbin.wu@linux.intel.com>]

On 10/10/2023 1:46 PM, kirill.shutemov@linux.intel.com wrote:
> On Tue, Oct 10, 2023 at 11:51:32AM +0800, Binbin Wu wrote:
>>
>> On 9/27/2023 7:02 PM, kirill.shutemov@linux.intel.com wrote:
>>> On Sun, Sep 24, 2023 at 07:33:46AM +0800, Binbin Wu wrote:
>>>> Zero out the buffer for readlink() since readlink() does not append a
>>>> terminating null byte to the buffer.
>>>>
>>>> Fixes: 833c12ce0f430 ("selftests/x86/lam: Add inherit test cases for linear-address masking")
>>>>
>>>> Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
>>>> ---
>>>>    tools/testing/selftests/x86/lam.c | 2 +-
>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/tools/testing/selftests/x86/lam.c b/tools/testing/selftests/x86/lam.c
>>>> index eb0e46905bf9..9f06942a8e25 100644
>>>> --- a/tools/testing/selftests/x86/lam.c
>>>> +++ b/tools/testing/selftests/x86/lam.c
>>>> @@ -680,7 +680,7 @@ static int handle_execve(struct testcases *test)
>>>>    		perror("Fork failed.");
>>>>    		ret = 1;
>>>>    	} else if (pid == 0) {
>>>> -		char path[PATH_MAX];
>>>> +		char path[PATH_MAX] = {0};
>>> Shouldn't it be PATH_MAX+1 to handle the case when readlink(2) stores
>>> exactly PATH_MAX bytes into the buffer?
>> According to the definition of PATH_MAX in include/uapi/linux/limits.h
>> #define PATH_MAX        4096    /* # chars in a path name including nul */
>>
>> IIUC, Linux limits the path length to 4095 and PATH_MAX includes the
>> terminating nul.
> Consider the case when kernel bump PATH_MAX to 8192. The binary that
> compiled from lam.c against the older kernel headers will get compromised.
>
> Increase the size of the buffer by one or pass PATH_MAX - 1 as buffer size
> to readlink(2).

Make sense, thanks!
I will send a new version as following:

diff --git a/tools/testing/selftests/x86/lam.c 
b/tools/testing/selftests/x86/lam.c
index eb0e46905bf9..8f9b06d9ce03 100644
--- a/tools/testing/selftests/x86/lam.c
+++ b/tools/testing/selftests/x86/lam.c
@@ -573,7 +573,7 @@ int do_uring(unsigned long lam)
         char path[PATH_MAX] = {0};

         /* get current process path */
-       if (readlink("/proc/self/exe", path, PATH_MAX) <= 0)
+       if (readlink("/proc/self/exe", path, PATH_MAX - 1) <= 0)
                 return 1;

         int file_fd = open(path, O_RDONLY);
@@ -680,14 +680,14 @@ static int handle_execve(struct testcases *test)
                 perror("Fork failed.");
                 ret = 1;
         } else if (pid == 0) {
-               char path[PATH_MAX];
+               char path[PATH_MAX] = {0};

                 /* Set LAM mode in parent process */
                 if (set_lam(lam) != 0)
                         return 1;

                 /* Get current binary's path and the binary was run by 
execve */
-               if (readlink("/proc/self/exe", path, PATH_MAX) <= 0)
+               if (readlink("/proc/self/exe", path, PATH_MAX - 1) <= 0)
                         exit(-1);

                 /* run binary to get LAM mode and return to parent 
process */


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