Actually, on further thoughts, even David's solution will require an extra check, if -E2BIG is returned. So, I guess the solution suggested by me looks the best (https://lore.kernel.org/linux-serial/868025b485b94480ad17d0ec971b3ee9@xxxxxxxxxxxxxxxx/T/#m1c4aaa4347b02fd4c11ce611ff5029fcb71c37a1) : 1. == Do not use the return value from strlcpy. == len = strlcpy(kbs, func_table[kb_func] ? : "", len); => strlcpy(kbs, func_table[kb_func] ? : "", len); 2. == Calculate the actual length of kbs, add 1, and then copy those many bytes to user-buffer == ret = copy_to_user(user_kdgkb->kb_string, kbs, len + 1) ? -EFAULT : 0; => ret = copy_to_user(user_kdgkb->kb_string, kbs, strlen(kbs) + 1) ? -EFAULT : 0; On Sun, Nov 7, 2021 at 12:50 AM Ajay Garg <ajaygargnsit@xxxxxxxxx> wrote: > > Thanks Pavel, Andy, David for the help. > > Andy, > > There is no compilation/runtime blocker. > There were warnings reported by smatch. > > My intention is to make the method "vt_do_kdgkb_ioctl" bullet-proof in > itself, without depending upon external clients. > > Pavel has explained that currently things are fine, as per : > https://lore.kernel.org/linux-serial/868025b485b94480ad17d0ec971b3ee9@xxxxxxxxxxxxxxxx/T/#m740fffb7c6ee52fdc98b9ef0b4e32a060b6a3be3 > > but it seems that there is a big flaw - we are dependent on the length > of "func_table[kb_func]" being ok. If func_table[kb_func] goes awry, > the method will cause overflow. > > Since func_table[kb_func]" is not managed by the method, so the method > must not depend on func_table[kb_func]" length-correctness. Instead, > "vt_do_kdgkb_ioctl" must ensure no overflow, without depending how > external entities (func_table[kb_func] behave. > > > > The issue with strlcpy, along with a potential "fix", has been explained in : > https://lore.kernel.org/linux-serial/868025b485b94480ad17d0ec971b3ee9@xxxxxxxxxxxxxxxx/T/#m1c4aaa4347b02fd4c11ce611ff5029fcb71c37a1 > > David has provided a simpler fix (usage of strscpy), as in : > https://lore.kernel.org/linux-serial/868025b485b94480ad17d0ec971b3ee9@xxxxxxxxxxxxxxxx/T/#m63dab1137e593f2030920a53272f71866b442f40 > > > So, we could go with one of the above changes (mine/David's), or > nothing at all (since there is no blocker). > > I vote for David's strscpy "fix", as it is simple, and does away with > the dependency on the length of "func_table[kb_func]". > > > Would like to know what the maintainers think. > If there is a consensus that the method "vt_do_kdgkb_ioctl" be made > bullet-proof in itself, please let me know, I will float the next > version of patch. > > > Thanks again Pavel, David, Andy. > > > Thanks and Regards, > Ajay
