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Date:   Sat, 28 Aug 2021 22:51:09 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     "Luck, Tony" <tony.luck@intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andreas Gruenbacher <agruenba@redhat.com>,
        Christoph Hellwig <hch@infradead.org>,
        "Darrick J. Wong" <djwong@kernel.org>, Jan Kara <jack@suse.cz>,
        Matthew Wilcox <willy@infradead.org>,
        cluster-devel <cluster-devel@redhat.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        ocfs2-devel@oss.oracle.com, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org
Subject: Re: [PATCH v7 05/19] iov_iter: Introduce fault_in_iov_iter_writeable
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Sender: Al Viro <viro@ftp.linux.org.uk>
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On Sat, Aug 28, 2021 at 10:19:04PM +0000, Al Viro wrote:

> How about taking __clear_user() out of copy_fpregs_to_sigframe()
> and replacing the call of fault_in_pages_writeable() with
> 	if (!clear_user(buf_fx, fpu_user_xstate_size))
> 		goto retry;
> 	return -EFAULT;
> in the caller?

Something like this (completely untested)

Lift __clear_user() out of copy_fpregs_to_sigframe(), do not confuse EFAULT with
X86_TRAP_PF, don't bother with fault_in_pages_writeable() (pointless, since now
__clear_user() on error is not under pagefault_disable()).  And don't bother
with retries on anything other than #PF...

diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
index 5a18694a89b2..71c6621a262f 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -17,6 +17,7 @@
 #include <linux/mm.h>
 
 #include <asm/user.h>
+#include <asm/trapnr.h>
 #include <asm/fpu/api.h>
 #include <asm/fpu/xstate.h>
 #include <asm/fpu/xcr.h>
@@ -345,7 +346,7 @@ static inline int xsave_to_user_sigframe(struct xregs_state __user *buf)
 	 */
 	err = __clear_user(&buf->header, sizeof(buf->header));
 	if (unlikely(err))
-		return -EFAULT;
+		return -X86_TRAP_PF;
 
 	stac();
 	XSTATE_OP(XSAVE, buf, lmask, hmask, err);
diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index 445c57c9c539..611b9ed9c06b 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -135,18 +135,12 @@ static inline int save_xstate_epilog(void __user *buf, int ia32_frame)
 
 static inline int copy_fpregs_to_sigframe(struct xregs_state __user *buf)
 {
-	int err;
-
 	if (use_xsave())
-		err = xsave_to_user_sigframe(buf);
-	else if (use_fxsr())
-		err = fxsave_to_user_sigframe((struct fxregs_state __user *) buf);
+		return xsave_to_user_sigframe(buf);
+	if (use_fxsr())
+		return fxsave_to_user_sigframe((struct fxregs_state __user *) buf);
 	else
-		err = fnsave_to_user_sigframe((struct fregs_state __user *) buf);
-
-	if (unlikely(err) && __clear_user(buf, fpu_user_xstate_size))
-		err = -EFAULT;
-	return err;
+		return fnsave_to_user_sigframe((struct fregs_state __user *) buf);
 }
 
 /*
@@ -205,9 +199,10 @@ int copy_fpstate_to_sigframe(void __user *buf, void __user *buf_fx, int size)
 	fpregs_unlock();
 
 	if (ret) {
-		if (!fault_in_pages_writeable(buf_fx, fpu_user_xstate_size))
+		if (!__clear_user(buf_fx, fpu_user_xstate_size) &&
+		    ret == -X86_TRAP_PF)
 			goto retry;
-		return -EFAULT;
+		return -1;
 	}
 
 	/* Save the fsave header for the 32-bit frames. */
@@ -275,7 +270,7 @@ static int restore_fpregs_from_user(void __user *buf, u64 xrestore,
 		fpregs_unlock();
 
 		/* Try to handle #PF, but anything else is fatal. */
-		if (ret != -EFAULT)
+		if (ret != -X86_TRAP_PF)
 			return -EINVAL;
 
 		ret = fault_in_pages_readable(buf, size);