Messages in this thread | | | Subject | Re: [PATCH v11 03/14] lib, arm64: untag user pointers in strn*_user | From | Kevin Brodsky <> | Date | Mon, 18 Mar 2019 11:33:14 +0000 |
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On 15/03/2019 19:51, Andrey Konovalov wrote: > This patch is a part of a series that extends arm64 kernel ABI to allow to > pass tagged user pointers (with the top byte set to something else other > than 0x00) as syscall arguments. > > strncpy_from_user and strnlen_user accept user addresses as arguments, and > do not go through the same path as copy_from_user and others, so here we > need to handle the case of tagged user addresses separately. > > Untag user pointers passed to these functions. > > Note, that this patch only temporarily untags the pointers to perform > validity checks, but then uses them as is to perform user memory accesses.
Thank you for this new version, looks good to me.
To give a bit of context to the readers, I asked Andrey to make this change, because it makes a difference with hardware memory tagging. Indeed, in that situation, it is always preferable to access the memory using the user-provided tag, so that tag checking can take place; if there is a mismatch, a tag fault will occur (which is handled in a way similar to a page fault). It is also preferable not to assume that an untagged user pointer (tag 0x0) bypasses tag checks.
Kevin
> > Signed-off-by: Andrey Konovalov <andreyknvl@google.com> > --- > lib/strncpy_from_user.c | 3 ++- > lib/strnlen_user.c | 3 ++- > 2 files changed, 4 insertions(+), 2 deletions(-) > > diff --git a/lib/strncpy_from_user.c b/lib/strncpy_from_user.c > index 58eacd41526c..6209bb9507c7 100644 > --- a/lib/strncpy_from_user.c > +++ b/lib/strncpy_from_user.c > @@ -6,6 +6,7 @@ > #include <linux/uaccess.h> > #include <linux/kernel.h> > #include <linux/errno.h> > +#include <linux/mm.h> > > #include <asm/byteorder.h> > #include <asm/word-at-a-time.h> > @@ -107,7 +108,7 @@ long strncpy_from_user(char *dst, const char __user *src, long count) > return 0; > > max_addr = user_addr_max(); > - src_addr = (unsigned long)src; > + src_addr = (unsigned long)untagged_addr(src); > if (likely(src_addr < max_addr)) { > unsigned long max = max_addr - src_addr; > long retval; > diff --git a/lib/strnlen_user.c b/lib/strnlen_user.c > index 1c1a1b0e38a5..8ca3d2ac32ec 100644 > --- a/lib/strnlen_user.c > +++ b/lib/strnlen_user.c > @@ -2,6 +2,7 @@ > #include <linux/kernel.h> > #include <linux/export.h> > #include <linux/uaccess.h> > +#include <linux/mm.h> > > #include <asm/word-at-a-time.h> > > @@ -109,7 +110,7 @@ long strnlen_user(const char __user *str, long count) > return 0; > > max_addr = user_addr_max(); > - src_addr = (unsigned long)str; > + src_addr = (unsigned long)untagged_addr(str); > if (likely(src_addr < max_addr)) { > unsigned long max = max_addr - src_addr; > long retval;
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